Multi-Layer Video Stream Encoding and Decoding

ABSTRACT

Multi-layer dependencies are signaled in an efficient way for a multi-view video stream (1). Coding or decoding relationship information defining coding or decoding dependencies is represented in bit-efficient syntax code, preferably through usage of hierarchical layer dependencies using layer indices for representing layer dependencies.

TECHNICAL FIELD

The embodiments generally relate to processing of a multi-layer videostream, and in particular to decoding and encoding of such a multi-layervideo stream.

BACKGROUND

HEVC (High Efficiency Video Coding) is the next generation video codingstandard that is currently under development in standardization. HEVCwill substantially improve coding efficiency compared to thestate-of-the-art H.264/Advanced Video Coding (AVC). The initial focus ofthe HEVC development was on mono video, i.e. one camera view, at a fixedquality and bit rate, i.e. non-scalable. Now multi-layer extensions tothe HEVC standard are under preparation, e.g. a scalable extension, amulti-view extension, and a 3D extension. Those extensions requiremulti-layer support. A HEVC bitstream without extensions can beconsidered as a single-layer bitstream, i.e. it represents the video ina single representation, e.g. a single video view, a single resolutionand a single quality. In multi-layer extensions, a HEVC single-layerbitstream is typically included as a “base layer”. In multi-view or 3Dextensions, additional layers may represent additional video viewscaptured from different camera positions or, for instance, depthinformation. In scalability extensions, additional layers may representthe video in additional, typically higher, video picture resolutions, orhigher pixel fidelity, or other color-spaces, or alike, providingimproved video quality relative to the base layer.

Specific decoders are used to decode HEVC bitstreams with multiplelayers, i.e. scalable or multi-view/3D HEVC decoders. In order to decodemulti-layer bitstreams, information about decoding dependencies betweenlayers is necessary. This information needs to be signaled in thebitstream. The information can also be used by network elements toidentify layers that can be discarded from the transmission if bit rateadaptation, e.g. in case of network congestion, or format adaptation,e.g. in case a target device can only decode or display a certainmaximum resolution, or 2D/3D adaptation, e.g. in case target device canonly decode or display a certain number of views, is needed.

The dependency information in HEVC is typically obtained in so-calledparameter sets, such as Picture Parameter Set (PPS), Sequence ParameterSet (SPS), or Video Parameter Set (VPS), together with otherinformation. Typically, each parameter set is encapsulated in a NetworkAbstraction Layer (NAL) unit, i.e. a packet in the video bitstream.Since parameter sets contain information that is essential for decoding,they may be sent repeatedly in the bitstream, or be conveyed by“out-of-band” transmission, i.e. transmitted separately from theremaining bitstream, e.g. over a reliable connection. Such anout-of-band transmission can occur, for instance during session setup,e.g. using Session Description Protocol (SDP).

If parameter sets are sent at session start-up, the amount of data inthe parameter set has an impact on the transmission duration and thussession start-up time. If parameter sets are sent “in-band”, i.e. in thebitstream, the size of parameter sets has an impact on the overallbitrate, and the impact is higher when the parameter sets are repeatedin the bitstream for error resiliency reasons. For these reasons it isimportant that the information conveyed in the parameter sets isexpressed in a compact way.

A document Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11 at 11^(th) Meeting: Shanghai, CN,10-19 Oct. 2012 denoted JCTVC-K1007 relating to NAL unit header andparameter set designs for HEVC extensions includes specifications ofparameter set designs for HEVC multi-view/3D and scalable codingextensions. According to that document, layer dependencies are signaledas part of the vps_extension syntax structure as indicated below:

vps_extension( ) { Descriptor . . . for( i = 1; i ≦vps_max_layers_minus1; i++ ) { //layer dependency num_direct_ref_layers[i ] u(6) for( j = 0; j < num_direct_ref_layers[ i ]; j++ ) ref_layer_id[i ][ j ] u(6) } } num_direct_ref_layers[ i ] specifies the number oflayers the i-th layer directly depends on. ref_layer_id[ i ][ j ]identifies the j-th layer the i-th layer directly depends on.

The above mentioned solution requires many bits to signal the layerdependencies in the VPS. In particular, for each layer above the baselayer in use, six bits are used to code the number of reference layersand another six bits are used to identify each reference layer. Thisallows signaling dependencies for relevant cases, however it may beinefficient in terms of bit usage.

SUMMARY

It is a general objective to provide an efficient processing of amulti-layer video stream.

It is a particular objective to signal layer dependencies in anefficient way for a multi-layer video stream.

These and other objectives are met by embodiments disclosed herein.

An aspect of the embodiments relates to a method of determining decodingrelationship for a coded multi-layer video stream defining multiplelayers of pictures. The method comprises retrieving, based on the codedmulti-layer video stream, at least one direct decoding flag indicating adirect coding relationship between a layer with layer index i of themultiple layers and a layer with layer index j of the multiple layers,i≠j. The method also comprises determining information defining anydecoding relationship between the multiple layers based on the at leastone direct decoding flag.

In an embodiment of this aspect the retrieving step comprisesretrieving, for the layer with layer index i, a respective directdependency flag for each layer index j based on the coded multi-layervideo stream. In this embodiment, j<i and the direct dependency flagindicates whether the layer with layer index j is a direct referencelayer for the layer with layer index i. In this embodiment thedetermining step comprises determining information defining any layerwith layer index j<i, on which the layer with layer index i depends,based on the direct dependency flags.

In an embodiment of this aspect the retrieving step comprisesretrieving, from a video parameter set or video parameter set extensionassociated with the coded multi-layer video stream, the at least onedirect decoding flag indicating the direct coding relationship betweenthe layer with layer index i and the layer with layer index j.

A related aspect of the embodiments defines a device for determiningdecoding relationship for a coded multi-layer video stream definingmultiple layers of pictures. The device comprises a flag retrieverconfigured to retrieve, based on the coded multi-layer video stream, atleast one direct decoding flag indicating a direct coding relationshipbetween a layer with layer index i of the multiple layers and a layerwith layer index j of the multiple layers, i≠j. The device alsocomprises an information determiner configured to determine informationdefining any decoding relationship between the multiple layers based onthe at least one direct decoding flag.

In an embodiment of this related aspect the flag retriever is configuredto retrieve, for the layer with layer index i, a respective directdependency flag for each layer index j based on the coded multi-layervideo stream. In this embodiment, j<i and the direct dependency flagindicates whether the layer with layer index j is a direct referencelayer for the layer with layer index i. In this embodiment theinformation determiner is configured to determine information definingany layer with layer index j<i, on which the layer with layer index idepends, based on the direct dependency flags.

In an embodiment of this related aspect the flag retriever is configuredto retrieve, from a video parameter set or video parameter set extensionassociated with the coded multi-layer video stream, the at least onedirect decoding flag indicating the direct coding relationship betweenthe layer with layer index i and the layer with layer index j.

A variant of this related aspect of the embodiments defines a device fordetermining decoding relationship for a coded multi-layer video streamdefining multiple layers of pictures. The device comprises a processorconfigured to process code means of a computer program stored in acomputer readable medium. The code means causes, when run on theprocessor, the processor to retrieve, based on the coded multi-layervideo stream, at least one direct decoding flag indicating a directcoding relationship between a layer with layer index i of the multiplelayers and a layer with layer index j of the multiple layers, i≠j. Thecode means also causes the processor to determine information definingany decoding relationship between the multiple layers based on the atleast one direct decoding flag.

Another aspect of the embodiments relates to a method of decoding acoded multi-layer video stream defining multiple layers of pictures,each layer of the multiple layers having a respective layer identifier.The method comprises retrieving, for a layer with a layer index of themultiple layers, decoding relationship information based on the codedmulti-layer video stream. The decoding relationship information definesa respective layer index of any reference layer of the multiple layers,on which the layer directly depends. The method also comprises mapping,for each reference layer and for the layer, its layer index to a layeridentifier based on mapping information of a hierarchical mappingrelationship between layer identifiers and layer indices. The mappinginformation is retrieved based on the coded multi-layer video stream.The method further comprises decoding a picture of the layer based on atleast one previously decoded picture in a layer of the multiple layersidentified based on the layer identifiers mapped from layer indices.

In an embodiment of this another aspect the retrieving step comprisesretrieving, based on the coded multi-layer video stream, at least onedirect decoding flag indicating a direct coding relationship between alayer with layer index i of the multiple layers and a layer with layerindex j of the multiple layers, i≠j. The retrieving step also comprises,in this embodiment, determining the decoding relationship informationbased on the at least one direct decoding flag.

In an embodiment of this another aspect the mapping step comprisesretrieving a flag vps_nuh_layer id_present_flag based on the codedmulti-layer video stream. The mapping step also comprises setting, foreach reference layer and for the layer and if vps_nuh_layerid_present_flag=0, its layer identifier equal to its layer index. Themapping step further comprises retrieving, for each reference layer andfor the layer and if vps_nuh_layer_id_present_flag=1, its layeridentifier from a vector layer_id_in_nuh[i], iε[1,vps_max_layers_minus1], wherein vps_max_layers_minus1+1 indicates amaximum number of layers and layer_id_in_nuh[i] indicates a layeridentifier for a layer with layer index i.

Another related aspect of the embodiments defines a decoder configuredto decode a coded multi-layer video stream defining multiple layers ofpictures, each layer of the multiple layers having a respective layeridentifier. The decoder comprises a decoding relationship informationretriever configured to retrieve, for a layer with a layer index of themultiple layers, decoding relationship information based on the codedmulti-layer video stream. The decoding relationship information definesa respective layer index of any reference layer of the multiple layers,on which the layer directly depends. An index-to-identifier mapping unitis configured to map, for each reference layer and for the layer, itslayer index to a layer identifier based on mapping information of ahierarchical mapping relationship between layer identifiers and layerindices. The mapping information is retrieved based on the codedmulti-layer video stream. The decoder further comprises a decoding unitconfigured to decode a picture of the layer based on at least onepreviously decoded picture in a layer of the multiple layers identifiedbased on the layer identifiers mapped from layer indices.

In an embodiment of this another related aspect the decodingrelationship information retriever comprises a flag retriever configuredto retrieve, based on the coded multi-layer video stream, at least onedirect decoding flag indicating a direct coding relationship between alayer with layer index i of the multiple layers and a layer with layerindex j of the multiple layers, i≠j. The decoding relationshipinformation retriever also comprises, in this embodiment, an informationdeterminer configured to determine the decoding relationship informationbased on the at least one direct decoding flag.

In an embodiment of this another related aspect the index-to-identifiermapping unit is configured to retrieve a flagvps_nuh_layer_id_present_flag based on the coded multi-layer videostream. The index-to-identifier mapping unit is also configured to set,for each reference layer and for the layer and ifvps_nuh_layer_id_present_flag=0, its layer identifier equal to its layerindex. The index-to-identifier mapping unit is further configured toretrieve, for each reference layer and for the layer and ifvps_nuh_layer_id_present_flag=1, its layer identifier from a vectorlayer_id_in_nuh[i], iε[1, vps_max_layers_minus1], whereinvps_max_layers_minus1+1 indicates a maximum number of layers andlayer_id_in_nuh[i] indicates a layer identifier for a layer with layerindex i.

A variant of this another related aspect of the embodiments defines adecoder configured to decode a coded multi-layer video stream definingmultiple layers of pictures, each layer of the multiple layers having arespective layer identifier. The decoder comprises a processorconfigured to process code means of a computer program stored in acomputer readable medium. The code means causes, when run on theprocessor, the processor to retrieve, for a layer with a layer index ofthe multiple layers, decoding relationship information based on thecoded multi-layer video stream. The decoding relationship informationdefines a respective layer index of any reference layer of the multiplelayers, on which the layer directly depends. The code means also causesthe processor to map, for each reference layer and for the layer, itslayer index to a layer identifier based on mapping information of ahierarchical mapping relationship between layer identifiers and layerindices. The mapping information is retrieved based on the codedmulti-layer video stream. The code means further causes the processor todecode a picture of the layer based on at least one previously decodedpicture in a layer of the multiple layers identified based on the layeridentifiers mapped from layer indices.

A further aspect of the embodiments relates to a method of determiningcoding relationship for a multi-layer video stream defining multiplelayers of pictures. The method comprises determining any codingrelationship between the multiple layers. The method also comprisesdetermining, for a layer with layer index i of the multiple layers andbased on the coding relationship, at least one direct decoding flagindicating a direct coding relationship between the layer with layerindex i and a layer with layer index j of the multiple layers, i≠j. Themethod further comprises associating the at least one direct decodingflag with a coded representation of the multi-layer video stream.

A further related aspect of the embodiments defines a device fordetermining coding relationship for a multi-layer video stream definingmultiple layers of pictures. The device comprises a relationshipdeterminer configured to determine any coding relationship between themultiple layers. The device also comprises a flag determiner configuredto determine, for a layer with layer index i of the multiple layers andbased on the coding relationship, at least one direct decoding flagindicating a direct coding relationship between the layer with layerindex i and a layer with layer index j of the multiple layers, i≠j. Thedevice further comprises an associating unit configured to associate theat least one direct decoding flag with a coded representation of themulti-layer video stream.

A variant of this further related aspect of the embodiments defines adevice for determining coding relationship for a multi-layer videostream defining multiple layers of pictures. The device comprises aprocessor configured to process code means of a computer program storedin a computer readable medium. The code means causes, when run on theprocessor, the processor to determine any coding relationship betweenthe multiple layers. The code means also causes the processor todetermine, for a layer with layer index i of the multiple layers andbased on the coding relationship, at least one direct decoding flagindicating a direct coding relationship between the layer with layerindex i and a layer with layer index j of the multiple layers, i≠j. Thecode means further causes the processor to associate the at least onedirect decoding flag with a coded representation of the multi-layervideo stream.

Yet another aspect of the embodiments relates to a method of encoding amulti-layer video stream defining multiple layers of pictures, eachlayer of the multiple layers having a respective layer identifier. Themethod comprises hierarchically mapping, for each layer of the multiplelayers, a layer identifier of the layer to a layer index based on codingdependencies between the multiple layers. The method also comprisesdetermining coding relationship information defining a respective layerindex of any reference layer of the multiple layers, on which a layer ofthe multiple layers directly depends. The method further comprisesgenerating a coded multi-layer video stream by encoding the pictures ofthe multiple layers based on the coding dependencies and associating thecoding relationship information with the coded multi-layer video stream.

In an embodiment of this another aspect the determining step comprisesdetermining any coding relationship between the multiple layers, anddetermining, for a layer with layer index i of the multiple layers andbased on the coding relationship, at least one direct decoding flagindicating a direct coding relationship between the layer with layerindex i and a layer with layer index j of the multiple layers, i≠j.

The associating step comprises, in this embodiment, associating the atleast one direct decoding flag with the coded multi-layer video stream.

Yet another related aspect of the embodiments defines an encoderconfigured to encode a multi-layer video stream defining multiple layersof pictures, each layer of the multiple layers having a respective layeridentifier. The encoder comprises a mapping unit configured tohierarchically map, for each layer of the multiple layers, a layeridentifier of the layer to a layer index based on coding dependenciesbetween the multiple layers. The encoder also comprises an informationdeterminer configured to determine coding relationship informationdefining a respective layer index of any reference layer of the multiplelayers, on which a layer of the multiple layers directly depends. Theencoder further comprises an encoding unit configured to generate acoded multi-layer video stream by encoding the pictures of the multiplelayers based on the coding dependencies and an associating unitconfigured to associate the coding relationship information with thecoded multi-layer video stream.

In an embodiment of this yet another related aspect the informationdeterminer comprises a relationship determiner configured to determineany coding relationship between the multiple layers, and a flagdeterminer configured to determine, for a layer with layer index i ofthe multiple layers and based on the coding relationship, at least onedirect decoding flag indicating a direct coding relationship between thelayer with layer index i and a layer with layer index j of the multiplelayers, i≠j. The associating unit is, in this embodiment, configured toassociate the at least one direct decoding flag with the codedmulti-layer video stream.

A variant of this yet another related aspect of the embodiments definesan encoder configured to encode a multi-layer video stream definingmultiple layers of pictures, each layer of the multiple layers having arespective layer identifier. The encoder comprises a processorconfigured to process code means of a computer program stored in acomputer readable medium. The code means causes, when run on theprocessor, the processor to hierarchically map, for each layer of themultiple layers, a layer identifier of the layer to a layer index basedon coding dependencies between the multiple layers. The code means alsocauses the processor to determine coding relationship informationdefining a respective layer index of any reference layer of the multiplelayers, on which a layer of the multiple layers directly depends. Thecode means further causes the processor to generate a coded multi-layervideo stream by encoding the pictures of the multiple layers based onthe coding dependencies. The code means additionally causes theprocessor to associate the coding relationship information with thecoded multi-layer video stream.

Further aspects of the embodiments define a user device and a networknode comprising a device for determining decoding relationship accordingto above and/or a decoder according to above above and a user devicecomprising a device for determining coding relationship according toabove and/or an encoder according to above.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments, together with further objects and advantages thereof,may best be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 schematically illustrates an example of a multi-layer videostream;

FIG. 2 schematically illustrates another example of a multi-layer videostream;

FIG. 3 illustrates an example of an encoded portion of a picture in amulti-layer video stream;

FIG. 4 illustrates an example of a data packet of a multi-layer videostream;

FIG. 5 is a flow diagram illustrating a method of decoding a codedmulti-layer video stream according to an embodiment;

FIG. 6 is a flow diagram illustrating a method of determining decodingrelationship for a coded multi-layer video stream according to anembodiment;

FIG. 7 is a flow diagram illustrating a method of determining decodingrelationship for a coded multi-layer video stream according to anotherembodiment;

FIG. 8 is a flow diagram illustrating an additional, optional step ofthe method in FIG. 7;

FIG. 9 is a flow diagram illustrating a method of encoding a multi-layervideo stream according to an embodiment;

FIG. 10 is a flow diagram illustrating a method of determining codingrelationship information for a multi-layer video stream according to anembodiment;

FIG. 11 is a flow diagram illustrating a method of determining codingrelationship information for a multi-layer video stream according toanother embodiment;

FIG. 12 is a schematic block diagram of a decoder according to anembodiment;

FIG. 13 is a schematic block diagram of a device for determiningdecoding relationship according to an embodiment;

FIG. 14 is a schematic block diagram of a device for determiningdecoding relationship according to another embodiment;

FIG. 15 is a schematic block diagram of an encoder according to anembodiment;

FIG. 16 is a schematic block diagram of a device for determining codingrelationship according to an embodiment;

FIG. 17 is a schematic block diagram of a device for determining codingrelationship according to another embodiment;

FIG. 18 is a schematic block diagram illustrating a particularimplementation example of devices according to the embodiments;

FIG. 19 is a schematic overview of encoding a multi-layer video stream,transmitting a coded multi-layer video stream, optional multi-layervideo sub-stream extraction and decoding and display of a multi-layervideo stream;

FIG. 20 is a diagram illustrating the number of bits required forsignaling layer dependencies when each layer with layer index i≧1 hasone reference layer;

FIG. 21 is a diagram illustrating the number of bits required forsignaling layer dependencies when each layer with layer index i≧2 hastwo reference layers and the layer with layer index i=1 has onereference layer;

FIG. 22 is a diagram illustrating the number of bits required forsignaling layer dependencies when each layer with layer index i≧1 hasthe maximum number of reference layers that are possible; and

FIG. 23 schematically illustrates another example of a multi-layer videostream.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similaror corresponding elements.

The present embodiments generally relate to multi-layer video streamsand sequences and in particular to encoding and decoding suchmulti-layer video streams. The embodiments provide an efficient way ofsignaling layer dependency for the multi-layer video stream, therebyenabling a reduction in a number of bits required to signal such layerdependencies as compared to prior art techniques.

Multi-layer video generally refers to multi-view video, 3D video and/orscalable video. In multi-view video a scene is captured from differentcamera views as indicated in FIG. 1 showing three such camera views. Themulti-layer video stream 1 then comprises, in this example, pictures 20,22, 24 from three different layers 10, 12, 14 or views. Pictures 20, 22,24 in the multi-layer video stream 1 can be encoded and decoded usingreference pictures belonging to the same layer 10, 12, 14 or view orbelonging to another layer 10, 12, 14 or view. FIG. 1 schematicallyillustrates the coding and decoding relationships between the picturesby arrows. For instance, picture 24 is encoded and decoded relative to aprevious, according to an encoding and decoding order, picture 22belonging to the same layer 12 or view. This picture 22 is, though,encoded and decoded using a previous picture of the same layer 12 orview and another picture 20 belonging to another layer 10 or view. InFIG. 1 the layer 10 or view is the base layer or view, whereas the otherlayers 12, 14 or views are additional layers or views, also known asenhancement layers or dependent views.

In 3D video the additional layer(s) may carry depth information or otherdata that could be used together with the pixel data of the base layerin order to generate 3D video.

In scalable video the additional layer(s) may represent the video inhigher video picture resolutions, higher pixel fidelity and/or othercolor-spaces in order to provide improved video quality relative to thebase layer of the scalable video.

FIG. 23 schematically illustrates an example of a multi-layer videostream 1 for scalable video. The figure indicates the organization ofthe pictures 20, 22, 24 in various scalable layers 10, 12, 14, wherelayer 10 indicates the base layer.

FIG. 2 schematically illustrates temporal scalability in which eachenhancement layer 12, 14 refines the frame rate.

A picture of a multi-layer video stream may be composed of one or moreslices. A slice is an independently decodable piece of the picture. Inother words, if one slice is lost, the other slices of that picture arestill decodable.

The encoding of a slice generates an encoded representation 30 of theslice comprising a slice header 32 and slice data 34 as is shown in FIG.3. The encoded presentation 30 is output from the encoding process as aso called Network Adaptation Layer (NAL) unit 44 as shown in FIG. 4. TheNAL unit 44 may be added with headers 42 to form a data packet 40 thatcan be transmitted as a part of a coded multi-layer video stream from anencoder to a decoder. For instance, Real-time Transport Protocol (RTP),User Datagram Protocol (UDP) and Internet Protocol (IP) headers 42 couldbe added to the NAL unit 44. This form of packetization of NAL units 44merely constitutes an example in connection with video transport. Otherapproaches of handling NAL units 44, such as file format, MPEG-2transport streams, MPEG-2 program streams, etc. are possible. FIG. 19schematically illustrates the generation of a coded multi-layer videostream 2 from an input multi-layer video stream 1 comprising multiplelayers 10, 12, 14 of pictures 20, 22, 24.

An aspect of the embodiments relates to a method of decoding a codedmulti-layer video stream 2 defining multiple layers 10, 12, 14 ofpictures 20, 22, 24. Each layer 10, 12, 14 of the multiple layers 10,12, 14 has a respective layer identifier. The method comprisesretrieving, for a layer 12 with a layer index of the multiple layers 10,12, 14, decoding relationship information based on the coded multi-layervideo stream 2. The decoding relationship information defines arespective layer index of any reference layer 10 of the multiple layers10, 12, 14 on which the layer 12 directly depends. The method alsocomprises mapping, for each reference layer 10 and for the layer 12, itslayer index to a layer identifier based on mapping information of ahierarchical mapping relationship between layer identifiers and layerindices. The mapping information is retrieved based on the codedmulti-layer video stream 2. The method further comprises decoding apicture 22 of the layer 12 based on at least one previously decodedpicture 20 in a layer 10 of the multiple layers 10, 12, 14 identifiedbased on the layer identifiers mapped from layer indices.

An embodiment of this aspect will now be further described withreference to the flow diagram of FIG. 5. FIG. 5 hence discloses a methodof decoding a coded multi-layer video stream, such as a multi-view videostream, a 3D video stream or a scalable video stream. The multi-layervideo stream defines or comprises multiple layers of pictures. Each suchlayer of the multi-layer video stream has a respective layer identifier.

The method typically starts in step S1 where decoding relationshipinformation is retrieved for a (current) layer of the multiple layers inthe multi-view video stream. The current layer for which the decodingrelationship information is retrieved has a layer index. Thus, thecurrent layer has, in addition to the above identified layer identifierthat is a general or “global” identifier for the current layer, a layerindex that is an index employed for signaling layer dependencies. Hence,the layer index is preferably configured to be used instead of the layeridentifier for the signaling of layer dependencies.

The decoding relationship information retrieved in step S1 defines arespective layer index of any reference layer of the multiple layers inthe multi-view video stream, on which the current layer directlydepends. Thus, the decoding relationship information defines arespective layer index of each reference layer that the current layerdirectly depends on.

As used herein a layer X directly depends (or is directly dependent) ona layer Y if e.g. pixel prediction from layer Y to layer X needs to beutilized to decode layer X. Thus, a picture belonging to layer X isencoded and thereby decoded based on a reference picture belonging tolayer Y.

Correspondingly, if a layer X directly depends (or is directlydependent) on a layer Y but does not directly depend (is not directlydependent) on a layer Z but the layer Y directly (or indirectly) depends(is directly or indirectly dependent) on the layer Z then the layer Xindirectly depends (is indirectly dependent) on the layer Z. A layer Xdepends (is dependent) on a layer Z if it either directly depends (isdirectly dependent) on the layer Z or indirectly depends (is indirectlydependent) on the layer Z.

If a layer X directly depends on a layer Y this layer Y is a directreference layer for the layer X.

With reference to FIG. 2 or 23, the layer with reference number 12directly depends on the base layer 10. The layer with reference number14 directly depends on the layer with reference number 12 and indirectlydepends on the base layer 10.

The decoding relationship information as retrieved in step S1 preferablylists the respective layer indices of the reference layers, on which thecurrent layer depends. Hence, also the reference layers have arespective layer index in addition to their respective layer identifier.

In an embodiment, the decoding relationship information is present in aparameter set included as a part of the multi-layer video stream, forinstance in a NAL unit, or provided out-of-band with regard to themulti-layer video stream. The parameter set preferably has an assignedparameter set identifier enabling identification of the relevantparameter set. Such a parameter set identifier is then preferablyincluded in the multi-layer video stream, such as present in a sliceheader of an encoded representation of a slice. Alternatively, the(first) parameter set identifier is included in a second parameter setwith a second parameter set identifier that is included in themulti-layer video stream e.g. in the slice header. In a furtheralternative, the (first) parameter set identifier is included in asecond parameter set with a second parameter set identifier that isincluded in a third parameter set that has a third parameter setidentifier that is included in the multi-layer video stream e.g. in theslice header.

In a first example, the decoding relationship information could beincluded in a VPS identified by a VPS identifier included in an SPS thatis identified by an SPS identifier included in a PPS having anassociated PPS identifier included in a slice header.

In a second example, the decoding relationship information could beincluded in an SPS that is identified by an SPS identifier included in aPPS having an associated PPS identifier included in a slice header.

In a third example, the decoding relationship information could beincluded in a PPS having an associated PPS identifier included in aslice header.

The decoding relationship information could, alternatively or inaddition, be included in a supplemental message, e.g. in a SupplementalEnhancement Information (SEI) message, such as in an active parametersets SEI message.

In another embodiment, the decoding relationship information is presentin multi-layer extension data included as a part of the multi-layervideo stream or provided out-of-band. Such multi-layer extension datacould be in the form of, for instance, multi-view extension data, 3Dextension data or scalable extension data. An alternative is to haveextension data relating to a parameter set, such as VPS extension data,PPS extension data or SPS extension data. Parameter set extension data,such as VPS extension data, SPS extension data, or PPS extension data,could be present as an optional extension of VPS, SPS, or PPS datastructures, e.g. depending on a flag. That is, a VPS/SPS/PPS could havean extension flag that indicates whether further data, extending theVPS/SPS/PPS, is present.

In a further embodiment, the decoding relationship information ispresent in the encoded representations of the coded multi-layer videostream. For instance, the decoding relationship information could bepresent in a respective slice header of one or more encodedrepresentations. In such a case, no identifiers to other data structuresof or associated with the multi-layer video stream is needed. However,this approach increases the total size in terms of the number of bits ofthe encoded representations that carries the mapping information.

A next step S2 of FIG. 5 comprises mapping the layer index to a layeridentifier for each reference layer defined by the decoding relationshipinformation as retrieved in step S1 and for the current layer. Thismapping of indices-to-identifiers in step S2 is performed based onmapping information of a hierarchical mapping relationship between layeridentifiers and layer indices. This mapping information is preferablyretrieved based on the coded multi-layer video stream. The mappinginformation could be retrieved from a parameter set, a parameter setextension, from another data structure of or associated with the codedmulti-layer video stream, from an encoded representation of a slice inthe coded multi-layer video stream, etc. The mapping information ispreferably provided in the same data structure as the decodingrelationship information but could, alternatively, be present in anotherdata structure.

The mapping information defines a hierarchical mapping relationshipbetween layer identifiers and layer indices. Hierarchical mappingindicates as used herein that the layer indices are assigned to thevarious layers in accordance with layer dependencies. In a particularembodiment, layers are indexed in a hierarchical way such that a layerwith a given layer index i can have only reference layers with lowerindices j, i.e. j<i. Thus, the respective layer index j assigned to thelayer(s) of the multiple layers in the multi-layer video stream that thecurrent layer with layer index i is dependent on is preferably lowerthan the layer index i. This further implies that a layer with layerindex i can have at most i reference layers, j=0, . . . , i−1.

The usage of layer indices instead of layer identifiers enables a morebit-efficient way of signaling layer dependencies. Generally, the layeridentifiers are, as indicated in the background section, of six bitseach for HEVC. The layer identifiers may, however, be sparsely assigned,i.e. some values of the 6-bits layer identifiers are never used. Thehierarchical mapping between layer identifiers and layer indicesaccording to an embodiment enables usage of layer indices that areassigned in a non-sparse way, i.e. all values of the layer indices areused.

The method as shown in FIG. 5 further comprises a step S3, whichcomprises decoding a picture of the current layer based on at least onepreviously decoded picture in a layer of the multiple layers in themulti-layer video stream, where this layer is identified based on thelayer identifiers mapped from the layer indices. Hence, the decodingrelationship information retrieved in step S1 is used to provide layerindices of any reference layers. The mapping information is then used instep S2 to map the respective layer index defined by the decodingrelationship information to a respective layer identifier and also mapthe layer index of current layer to a layer identifier. A picture, ormore correctly at least one picture, in the current layer is thendecoded based on at least one previously decoded picture, i.e. areference picture, in a layer identified based on at least one of thelayer identifiers obtained in the mapping performed in step S2. Thereference picture used in the decoding step S3 could be a referencepicture belonging to the current layer or to another layer of themulti-layer video stream. If more than one reference picture is to beused as decoding basis for the picture to be decoded in step S3 thesereference pictures could belong to the same or different layers of themulti-layer video stream.

The layer indices employed by the embodiments to signal layerdependencies may be different from the layer identifiers. The number oflayers is typically less than the number of different possible layeridentifiers, such as 64 possible layer identifiers for a 6-bit layeridentifier representation. Thus, the number of bits employed forsignaling a layer index is typically lower than the number of bits usedto signal a layer identifier. Furthermore, whereas a layer identifier isrepresented by a syntax element of a fixed number of bits, such as inu(6) code, requiring six bits for representing the layer identifier, alayer index is preferably represented by a syntax element of a variablelength, such as in u(v) code. Therein, the number v of bits used forindicating a layer index may be determined based on the maximum numberof layers used, for example as v=ceil(log 2(max_layers)), wheremax_layers indicates the maximum number of layers used, i.e.max_layers=vps_max_layers_minus1+1, log 2(x) indicates a base-2logarithm of x and ceil(y) indicates the smallest integer number greaterthan or equal to y.

In an implementation example, the following syntax elements could beused to indicate the decoding relationship information and the mappinginformation.

vps_extension( ) { Descriptor . . . for( i = 1; i ≦vps_max_layers_minus1; i++ ) { layer_id_in_nuh[ i ] u(6) } . . . for( i= 1; i ≦ vps_max_layers_minus1; i++ ) { num_direct_ref_layers[ i ] u(v)for( j = 0; j < num_direct_ref_layers[ i ]; j++) ref_layer_index[ i ][ j] u(v) } }

In the first loop i represents the layer index and layer_id_in_nuh[i]indicates the layer identifier for layer index i. Hence, these syntaxelements constitute the mapping information. In the second loopnum_direct_ref_layers[i] specifies the number of layers the layer withlayer index i directly depends on. ref_layer_index[i][j] identifies thelayer index of the j-th layer the layer with layer index i directlydepends on. In this example implementation a hierarchical layerrelationship is used such that a layer with index i can only havereference layers that have layer indices j<i.

The mapping information as used in step S2 and the decoding relationshipinformation retrieved in step S1 together enables an efficient signalingof layer dependencies by exploiting a hierarchical layer relationshipand a dense assignment of layer indices. As a consequence, the number ofbits required for signaling layer dependencies can be reduced ascompared to using layer identifiers.

Thus, the usage of layer indices for signaling layer dependenciesinstead of the layer identifiers and the usage a hierarchical layerindexing syntax elements or codewords of minimal length can be used fordefining the decoding relationship information.

In certain embodiments the layer indices could be the same as the layeridentifiers, i.e. layer_index=layer_id. However, in such a case thelayer identifiers are preferably hierarchically organized with regard tolayer dependencies as disclosed herein. In such a case, the VPSextension data disclosed above could be represented as shown below:

vps_extension( ) { Descriptor . . . vps_nuh_layer_id_present_flag u(1)for( i = 1; i ≦ vps_max_layers_minus1; i++ ) { if(vps_nuh_layer_id_present_flag ) layer_id_in_nuh[ i ] u(6) } . . . }

If vps_nuh_layer id_present_flag is equal to zero then there is noexplicit mapping between layer identifiers and layer indices. In such acase, it could be implicitly inferred that layer_id_in_nuh[i]=i.

Hence, in an embodiment the mapping step S2 in FIG. 5 comprisesretrieving a flag vps_nuh_layer id_present_flag based on the codedmulti-layer video stream, such as from a VPS extension as indicatedabove. The mapping step S2 further comprises setting, for each referencelayer and for the layer and if vps_nuh_layer id_present_flag=0, itslayer identifier equal to its layer index. The mapping step S2 alsocomprises retrieving, for each reference layer and for the layer and ifvps_nuh_layer id_present_flag=1, its layer identifier from a vectorlayer_id_in_nuh[i], iε[1, vps_max_layers_minus1], whereinvps_max_layers_minus1+1 indicates a maximum number of layers andlayer_id_in_nuh[i] indicates the layer identifier for the layer withlayer index i.

Another aspect of the embodiments relates to a method of encoding amulti-layer video stream 1 defining multiple layers 10, 12, 14 ofpictures 20, 22, 24. Each layer 10, 12, 14 of the multiple layers 10,12, 14 has a respective layer identifier. The method compriseshierarchically mapping, for each layer 10, 12, 14 of the multiple layers10, 12, 14, a layer identifier of the layer 10, 12, 14 to a layer indexbased on coding dependencies between the multiple layers 10, 12, 14. Themethod also comprises determining coding relationship informationdefining a respective layer index of any reference layer 10 of themultiple layers 10, 12, 14, on which a layer 12 of the multiple layers10, 12, 14 directly depends. The method further comprises generating acoded multi-layer video stream 2 by encoding the pictures 20, 22, 24 ofthe multiple layers 10, 12, 14 based on the coding dependencies andassociating the coding relationship information with the codedmulti-layer video stream 2.

FIG. 9 is a flow diagram illustrating an embodiment of the method ofencoding a multi-layer video stream. The method starts in step S40,which comprises hierarchically mapping a respective layer identifier ofeach layer in the multi-layer video stream to a respective layer index.This mapping between layer identifiers and layer indices is performedbased on coding dependencies between the multiple layers of themulti-layer video stream. Thus, the assignment of layer indices to thelayers is conducted based on how the various layers depend on eachother. In particular, the assignment of the layer indices during themapping in step S40 is performed such as that a layer with assignedlayer index i can only have reference layers, i.e. depend on layers,that have layer indices j, where j is smaller than i. Thus, giveninformation of how the layers of the multi-view video stream depend oneach other the layer identifiers of the layer are mapped to layerindices to form the hierarchical layer relationship.

Step S41 of the method comprises determining coding relationshipinformation defining a respective layer index of any reference layerwhich a current layer of the multi-layer video stream directly dependson. Thus, the coding relationship information specifies the layer indexor indices of all the reference layers for the current layer.

A coded multi-view video stream is generated in step S42 by encoding thepictures of the multiple layers based on and according to the codingdependencies between the multiple layers. Thus, P or B slices in thevarious layers are encoded using one or more reference picturesbelonging to the same layer as the P or B slice or belonging to one ofthe reference layers for the layer of the P or B slice. The picturecould, alternatively, be an I picture. In such a case, referencepictures are not handled in the same way as normal “temporal” referencepictures. Still an I picture, e.g. in a scalable HEVC extension, couldhave a reference picture, for “inter-layer prediction”.

The coded multi-view video stream preferably comprises respectiveencoded representations 30 of slices as shown in FIG. 3 for each sliceof the multi-layer video stream. The encoded representations 30 areadvantageously packed into NAL units 44 as shown in FIG. 4. The codedmulti-layer video stream may also comprise NAL units 44 comprising otherdata, such as parameter sets, than the encoded representations 30.

The coding relationship information determined in step S41 isassociated, in step S43, with the coded multi-layer video streamgenerated in step S42. This step S43 could be performed prior to stepS42 or at least partly in parallel with step S42. The association of thecoding relationship information with the coded multi-layer video streamcould be performed according to various embodiments as previouslydisclosed herein. For instance, the coding relationship informationcould be present in a parameter set in which case a parameter setidentifier is included in a slice header of at least one encodedrepresentation of a slice in the coded multi-layer video stream or inanother parameter set that is identifiable based on data included in theslice header. Alternatively, the coding relationship information couldbe present in a parameter set extension or indeed in encodedrepresentations of slices.

In a particular embodiment mapping information employed in step S40 tohierarchically map a respective layer identifier to a respective layerindex is preferably generated and associated with the coded multi-layervideo stream. In such a case, this mapping information is preferablyprovided in the same data structure as the coding relationshipinformation as previously discussed herein.

Aspects of the embodiments also relate to methods of determiningdecoding relationships for a coded multi-layer video stream anddetermining coding relationships for a multi-layer video stream. Theseembodiments could advantageously be applied to the method of decoding asshown in FIG. 5 or the method of encoding as shown in FIG. 9.Alternatively, these methods of determining coding/decodingrelationships could be used separately from the method of FIG. 5 and themethod of FIG. 9.

If these methods of determining coding/decoding relationships are usedseparated from the method of FIG. 5 and the method of FIG. 9, themethods could either use layer indices for the multiple layers in themulti-layer video stream or layer identifiers in the multi-layer videostream. Thus, in the latter case no explicit mapping of layeridentifiers into layer indices is necessary. For such instances, layerindex/layer indices as used herein with reference to the methods ofdetermining coding/decoding relationships can be replaced by layeridentifier/layer identifiers. It is then particularly preferred that thelayer identifiers are hierarchically organized.

An aspect of the embodiments relates to a method of determining decodingrelationship for a coded multi-layer video stream 2 defining multiplelayers 10, 12, 14 of pictures 20, 22, 24. The method comprisesretrieving, based on the coded multi-layer video stream 2, at least onedirect decoding flag indicating a direct coding relationship between alayer 12 with layer index i of the multiple layers 10, 12, 14 and alayer 10 with layer index j of the multiple layers 10, 12, 14, i≠j. Themethod also comprises determining information defining any decodingrelationship between the multiple layers 10, 12, 14 based on the atleast one direct decoding flag.

FIG. 6 is a flow diagram illustrating an embodiment of this method ofdetermining decoding relationship for a coded multi-layer video streamdefining multiple layers of pictures. The method starts in step S10,which comprises retrieving at least one direct decoding flag based onthe coded multi-layer video stream. The at least one direct decodingflag could be present in a parameter set, such as PPS, SPS or VPS, aparameter set extension, such as PPS extension, SPS extension or VPSextension, be present in an encoded representation of a slice, such asin the slice header, of the coded multi-layer video stream, or elsewherebeing provided in a data structure of or associated with the codedmulti-layer video stream. Thus, the coded multi-layer video streameither comprises the at least one direct decoding flag or comprisesinformation, such as an identifier, allowing identification of the datastructure comprising the at least one direct decoding flag. Theretrieval in step S10 could therefore be performed by parsing anddecoding a parameter set or parameter set extension to obtain the directdecoding flag. Alternatively and if the parameter set or parameter setextension already has been parsed and decoded and the data of theparameter set or parameter set extension is stored in a decoded form ina memory, step S10 could comprise reading or retrieving the at least onedirect decoding flag from such a memory. If the at least one directdecoding flag is present in an encoded representation of a slice stepS10 preferably retrieves the at least one direct decoding flag inconnection with parsing and decoding the slice header in the encodedrepresentation.

The at least one direct decoding flag retrieved in step S10 indicates adirect coding relationship between a layer with layer index i (currentlayer) of the multiple layers and a layer with layer index j of themultiple layers, where i≠j. Thus, the at least one direct decoding flagindicates whether there is any direct coding and thereby decodingrelationship between the layers with layer indices i and j. In aparticular embodiment, the direct decoding flag indicates whether thelayer with layer index j is a direct reference layer for the layer withlayer index i.

The at least one direct decoding flag is preferably a 1-bit flag, i.e.can assume either the value 0_(bin) or the value 1_(bin). In such acase, a direct decoding flag set to 1_(bin) could indicate that thelayer with layer index j is a direct reference layer for the layer withlayer index i, whereas a direct decoding flag set to 0_(bin) insteadindicates that the layer with layer index j is not a direct referencelayer for the layer with layer index i.

The method also comprises determining, in step S11, information definingany decoding relationship between the multiple layers in the multi-layervideo stream based on the at least one direct decoding flag retrieved instep S10. Thus, the at least one direct decoding flag retrieved in stepS10 is used to determine the information defining the decodingrelationship between the various layers in the multi-layer video stream.

The method then ends in an embodiment.

In an alternative embodiment the information determined in step S11 isused as the decoding relationship information in the method of decodinga coded multi-layer video stream described in connection with FIG. 5. Insuch a case, the method continues from step S11 of FIG. 6 to step S2 ofFIG. 5.

This means that in this alternative embodiment the layer indices arehierarchically assigned to the various layers so that a layer with layerindex i has any reference layers that have layer indices j<i. In such acase, the direct decoding flags enable a very compact and efficient wayof signaling layer dependencies for the multi-layer video stream.

The determination of the information in step S11 could, which is furtherdiscussed below, involve generating or calculating the information usingthe at least one direct decoding flag retrieved in step S10. In analternative embodiment, the determination of the information definingany decoding relationship between the multiple layers could simply bethe determination of the value(s) of the at least one direct decodingflag. Thus, in this case the respective value of the at least one directdecoding flag as decoded could constitute the information determined orprovided in step S11.

In an embodiment of step S10 a respective direct dependency flag isretrieved for the layer with layer index i for each layer index j basedon the coded multi-layer video stream. In this embodiment j<i and thedirect dependency flag indicates whether the layer with layer index j isa direct reference layer for the layer with layer index i.

Step S11 preferably comprises, in this embodiment, determininginformation defining any layer with layer index j<i which the layer withlayer index i depends on based on the direct dependency flag(s)retrieved in step S10. Thus, the direct dependency flag(s) obtained instep S10 is (are) used in order to determine the information definingthe layer(s) with respective layer index lower than the layer index ofthe current layer (layer index i) which the current layer directly orindirectly depends on.

In an embodiment step S10 comprises retrieving, form a video parameterset or a video parameter set extension associated with the codedmulti-layer video stream, the at least one direct decoding flagindicating the direct coding relationship between the layer with layerindex i and the layer with layer index j.

In a particular embodiment step S10 preferably comprises retrieving, forthe layer with layer index i, a respective direct dependency flagdirect_dependency_flag[i][j] for each layer index j based on the codedmulti-layer video stream. In this particular embodiment step S11preferably comprises determining a dependency flag set or arraydependency_flag[i] for the layer with layer index i. This dependencyflag set is preferably determined asdependency_flag[i]=direct_dependency_flag[i]+sum_(j=0 . . . i−1)(direct_dependency_flag[i][j]×dependency_flag[j]).

-   -   direct_dependency_flag[i] represents a vector, i.e. a        concatenation of elements in direct_dependency_flag[i][j];    -   “+” represents a bit-wise logical OR operation;    -   sum_(j=0 . . . i−1)(x(j)) represents a bit-wise logical OR        operation over all x(j) with j=0 . . . i−1; and    -   “x” represents a bit-wise AND operation between each element in        dependency_flag[j] and direct_dependency_flag[i][j]

In such a case, the syntax elements for signaling the direct dependencyflags could be as defined below.

for( i = 1; i ≦ vps_max_layers_minus1; i++ ) { Descriptor // layerdependency for( j = 0; j < i; j++ ) direct_dependency_flag[ i ][ j ]u(1) }

direct_dependency_flag[i][j] equal to zero preferably indicates that thelayer with layer index j is not a direct reference layer for the layerwith layer index i. direct_dependency_flag[i][j] equal to one preferablyindicates that the layer with layer index j is a direct reference layerfor the layer with layer index i. If not presentdirect_dependency_flag[i][j] is preferably inferred to be equal to zero.

vps_max_layers_minus1+1 indicates the maximum possible number of layersin the coded multi-layer video stream.

If this embodiment is used in connection with the method of decoding asshown in FIG. 5 the direct dependency flags and the mapping informationcould be signaled together, such as in a VPS extension as shown below.

vps_extension( ) { Descriptor . . . for( i = 1; i ≦vps_max_layers_minus1; i++ ) { layer_id_in_nuh[ i ] u(6) } . . . for( i= 1; i ≦ vps_max_layers_minus1; i++ ) { for( j = 0; j < i; j++ )direct_dependency_flag[ i ][ j ] u(1) } }

In another embodiment step S10 of FIG. 6 comprises retrieving, for alayer with layer index i, a respective direct reference flag for eachlayer index j based on the coded multi-layer video stream. In thisembodiment j>i and the direct reference flag indicates whether the layerwith layer index i is a direct reference layer for the layer with layerindex j. Step S11 preferably comprises determining information definingany layer with layer index j>i for which the layer with layer index i isa (direct or indirect) reference layer based on the direct referenceflag(s) retrieved in step S10.

In a particular embodiment step S10 preferably comprises retrieving, forthe layer with layer index i, a respective direct reference flagdirect_reference_flag[i][j] for each layer index j based on the codedmulti-layer video stream. Step S11 preferably comprises determining areference flag set or array reference_flag[i] for the layer with layerindex i. This reference flag set is preferably determined asreference_flag[i]=direct_reference_flag[i]+sum_(j=i+1 . . . imax)(direct_reference_flag[i][j]×reference_flag[j]).

-   -   reference_flag[i] represents a vector, i.e. a concatenation of        elements in reference_flag[i][j];    -   direct_reference_flag[i] represents a vector, i.e. a        concatenation of elements in direct_reference_flag[i][j];    -   “+” represents a bit-wise logical OR operation;    -   sum_(j+1 . . . imax)(x(j)) represents a bit-wise logical OR        operation over all x(j) with j=i+1 . . . imax, where imax        represents the maximum layer index; and    -   “x” represents a bit-wise AND operation between each element in        reference_flag[j] and direct_reference_flag[i][j]

In such a case, the syntax elements for signaling the direct dependencyflags could be as defined below.

for( i = 1; i ≦ vps_max_layers_minus1; i++ ) { Descriptor // layerdependency for( j = i+1; j ≦ vps_max_layers_minus1; j++ )direct_reference_flag[ i ][ j ] u(1) }

direct_reference_flag[i][j] equal to zero preferably indicates that thelayer with layer index i is not a direct reference layer for the layerwith layer index j. direct_reference_flag[i][j] equal to one preferablyindicates that the layer with layer index i is a direct reference layerfor the layer with layer index j. If not presentdirect_reference_flag[i][j] is preferably inferred to be equal to zero.

If this embodiment is used in connection with the method of decoding asshown in FIG. 5 the direct reference flags and the mapping informationcould be signaled together, such as in a VPS extension as shown below.This embodiment then preferably has a hierarchical layer relationshipsince a layer with layer index i can preferably only be a referencelayer for layers with layer index j>i.

vps_extension( ) { Descriptor . . . for( i = 1; i ≦vps_max_layers_minus1; i++ ) { layer_id_in_nuh[ i ] u(6) } . . . for( i= 1; i ≦ vps_max_layers_minus1; i++ ) { for( j = i+1; j ≦vps_max_layers_minus1; j++ ) direct_reference_flag[ i ][ j ] u(1) } }

In a further embodiment step S10 of FIG. 6 comprises retrieving, for alayer with layer index i, a respective direct dependency flagdirect_dependency_flag[i][j] for each layer index j based on said codedmulti-layer video stream.

In a particular embodiment direct_dependency_flag[i][j] equal to 0indicates that the layer with layer index j is not a direct referencelayer for the layer with layer index i. Correspondingly,direct_dependency_flag[i][j] equal to 1 indicates that the layer withlayer index j may be a direct reference layer for the layer with layerindex i.

Step S11 of FIG. 6 comprises in this embodiment determining variablesNumDirectRefLayers[i] and RefLayerId[i][j], respectively, based on thedirect dependency flags direct_dependency_flag[i][j].NumDirectRefLayers[i] is a variable represents the number of directreference layers for the layer with layer index i and the variableRefLayerId[i][j] represents the layer identifier of the jth directreference layer for the layer with layer index i.

In a particular embodiment, the variables NumDirectRefLayers[i] andRefLayerId[i][j] are derived in step S11 according to:

for( i = 1; i ≦ vps_max_layers_minus1; i++ ) for( j = 0;NumDirectRefLayers[ i ] = 0; j < i; j++ ) if( direct_dependency_flag[ i][ j ] == 1) RefLayerId[ i ][ NumDirectRefLayers[ i ]++ ] =layer_id_in_nuh[ j ]

If this embodiment is used in connection with the method of decoding asshown in FIG. 5 the direct reference flags and the mapping informationcould be signaled together, such as in a VPS extension as shown below.This embodiment then preferably has a hierarchical layer relationshipsince a layer with layer index i can preferably only be a referencelayer for layers with layer index j>i.

vps_extension( ) { Descriptor . . . for( i = 1; i ≦vps_max_layers_minus1; i++ ) { layer_id_in_nuh[ i ] u(6) } . . . for( i= 1; i ≦ vps_max_layers_minus1; i++ ) { for( j = i+1; j ≦vps_max_layers_minus1; j++ ) direct_reference_flag[ i ][ j ] u(1) } }

An aspect of the embodiments relates to a method of determining decodingrelationship for a coded multi-layer video stream 2 defining multiplelayers 10, 12, 14 of pictures 20, 22, 24. The method optionallycomprises calculating a length of a number syntax element. The methodalso comprises retrieving, based on the coded multi-layer video stream2, the number syntax element defining a number of reference layers ofthe multiple layers 10, 12, 14 for a layer 12 with layer index i of themultiple layers 10, 12, 14, optionally based on the length of the numbersyntax element. The method further comprises retrieving, based on thecoded multi-layer video stream 2 and for each of the number of referencelayers, a respective reference syntax element defining a layer index jof a layer 10 of the multiple layers 10, 12, 14 that the layer 12 withlayer index i directly depends on, j<i.

FIG. 7 is a flow diagram illustrating an embodiment of this method ofdetermining decoding relationship for a coded multi-layer video streamdefining multiple layers of pictures. The method starts in an optionalstep S20, which comprises calculating a length of a number syntaxelement. This number syntax element preferably defines a number ofreference layers for a current layer with layer index i.

In a particular embodiment the length of the number syntax element interms of number of bits is variable and depends on at least one inputparameter or syntax element as further discussed below. In such a case,the length of the number syntax element could be calculated in step S20to determine how many bits the number syntax element encompasses. It ispossible that the number syntax element has a length common for multiplelayers, such as all layers, in the multi-layer video stream. In such acase, the length of the number syntax element could be calculated oncein step S20 for a first layer. This calculated length value could thenbe stored and reused for the other layers in the multi-layer videostream, thereby relaxing the need for performing step S20 for each layerin the multi-layer video stream.

Step S21 of FIG. 7 comprises retrieving the number syntax element basedon the coded multi-layer video stream. The number syntax element couldbe retrieved from a parameter set, a parameter set extension, any otherdata structure of or associated with the coded multi-layer video streamor from an encoded representation of the slice in the coded multi-layervideo stream.

In an embodiment, step S21 comprises retrieving the number syntaxelement based on the length of the number syntax element as calculatedin the optional step S20. Thus, information of the length of the numbersyntax element is employed in step S21 in order to identify the portion,i.e. the number of bits, in the relevant structure that corresponds tothe number syntax element.

In an alternative approach, the retrieval of the number syntax elementis performed in step S21 without using any calculated length of thenumber syntax element. For instance, the number syntax element could beidentified based on parsing and decoding the data structure, such asstarting from the beginning of the data structure.

The method then continues to step S22 where a respective referencesyntax element is retrieved based on the coded multi-layer video streamand for each of the number of reference layers as defined by the numbersyntax element retrieved in step S21. The reference syntax element(s)retrieved in step S22 could be retrieved from a parameter set, aparameter set extension, any other data structure of or associated withthe coded multi-layer video stream or from an encoded representation ofthe slice in the coded multi-layer video stream. In a particularembodiment, the reference syntax element(s) is (are) preferably presentin the same data structure as the number syntax element.

A reference syntax element defines a layer index j of a layer in themulti-layer video stream that the current layer with layer index idirectly depends on. In addition, in a preferred embodiment ahierarchical layer relationship is induced such that j<i.

The number syntax element, thus, signals how many reference layers thecurrent layer with layer index i directly depends on. This number syntaxelement is therefore employed in step S22 in order to know how manyreference syntax elements to retrieve for the layer with layer index i.Each such reference syntax element retrieved in step S22 defines arespective layer index j for a layer that the current layer with layerindex i directly depends on.

The method then ends in an embodiment.

In an alternative embodiment the reference syntax elements retrieved instep S22 are used as the decoding relationship information in the methodof decoding a coded multi-layer video stream described in connectionwith FIG. 5. In such a case, the method continues from step S22 of FIG.7 to step S2 of FIG. 5.

In a first particular embodiment the optional step S20 comprisescalculating the length of the number syntax element based on the layerindex i. Thus, in this embodiment the length of the number syntaxelement is calculated based on the particular value of the layer indexi.

In a first example implementation step S20 comprises calculating thelength of the number syntax element as ceil(log 2(i+1)), wherein irepresents the layer index, ceil(x) represents a smallest integer numbergreater than or equal to x and log 2(x) represents a base-2 logarithm ofx.

Consequently, in this example implementation the number of bits used torepresent the number of reference layers, i.e. the number syntaxelement, is represented by a u(v) code with v=ceil(log 2(i+1)).

In a second particular embodiment the optional step S20 comprisescalculating the length of the number syntax element based on informationof a maximum possible number of layers in the coded multi-layer videostream.

In a second example implementation step S20 comprises calculating thelength of the number syntax element as ceil(log2(vps_max_layers_minus1+1)), wherein vps_max_layers_minus1+1 indicates,as previously disclosed herein, the maximum number of layers in thecoded multi-layer video stream.

This second example implementation would be slightly less bit efficientthan the first example implementation. However, an advantage is that thecode length of the number syntax element is independent on theparticular layer (layer index), which may lead to a less complexdecoding of the syntax structure.

In a third particular embodiment the optional step S20 comprisescalculating the length of the number syntax element based on informationof a maximum value of a layer identifier in the coded multi-layer videostream.

In a third example implementation step S20 comprises calculating thelength of the number syntax element as ceil(log 2(vps_max_layer_id−1)),wherein vps_max_layer_id indicates the maximum possible value for thelayer identifiers (layer_id) in the coded multi-layer video stream.

In an optional embodiment the method as shown in FIG. 7 comprises anadditional step S30, see FIG. 8. In such a case the method continuesfrom step S21 of FIG. 7. Step S30 comprises calculating a respectivelength of the respective reference syntax elements to be retrieved instep S22 of FIG. 7.

In an embodiment, step S30 comprises calculating the respective lengthof the respective reference syntax element based on the layer index i,such as based on the particular value of this layer index i.

In a first implementation example the number of bits used to representthe reference syntax element(s) is equal to ceil(log 2(i)).

In such a case, the syntax elements for signaling the number syntaxelement and the reference syntax elements could be as defined below.

for( i = 1; i ≦ vps_max_layers_minus1; i++ ) { Descriptornum_direct_ref_layers[ i ] u(v) for( j = 0; j < num_direct_ref_layers[ i]; j++) ref_layer_index[ i ][ j ] u(v) }

num_direct_ref_layers[i] specifies the number of reference layer thelayer with layer index i directly depends on. The number of bits used torepresent num_direct_ref_layers[i] could be v=ceil(log 2(i+1)) orv=ceil(log 2(vps_max_layers_minus1+1)) or v=ceil(log2(vps_max_layer_id−1)). ref_layer_index[i][j] identifies the layer indexof the j-th layer the layer with layer index i directly depends on. Thenumber of bits used to represent ref_layer_index[i][j] is preferablyequal to v=ceil(log 2(i)).

If this embodiment is used in connection with the method of decoding asshown in FIG. 5 the number syntax elements, the reference syntaxelements and the mapping information could be signaled together, such asin a VPS extension as shown below. This embodiment then preferably has ahierarchical layer relationship since a layer with layer index i canpreferably only be a reference layer for layers with layer index j>i.

vps_extension( ) { Descriptor . . . for( i = 1; i ≦vps_max_layers_minus1; i++ ) { layer_id_in_nuh[ i ] u(6) } . . . for( i= 1; i ≦ vps_max_layers_minus1; i++ ) { num_direct_ref_layers[ i ] u(v)for( j = 0; j < num_direct_ref_layers[ i ]; j++) ref_layer_index[ i ][ j] u(v) } }

In a second implementation example an assumption is made that thereference indices, i.e. the reference syntax elements, are indicated ina hierarchical order of layer indices for the reference layers. Forinstance, it can be assumed thatref_layer_index[i][j]>ref_layer_index[i][j−1] for j>0, whereinref_layer_index[i][j] represents the layer index of the j-th layer thatthe layer with layer index directly depends on. Then the j-th referencelayer index for the layer with layer index i can be represented as adelta layer index, delta_ref_layer_index[i][j], such thatref_layer_index[i][j]=ref_layer_index[i][j−1]+1+delta_ref_layer_index[i][j].In such a case, ref_layer_index[i][−1]=−1 anddelta_ref_layer_index[i][j]≧0. With a hierarchical layer relationshipref_layer_index[i][j]<i and, thus,0≦delta_ref_layer_index[i][j]<i−ref_layer_index[i][j−1]−1. This in turnimplies that delta_ref_layer_index[i][j] can be represented by a u(v)code with v=ceil(log 2(i−ref_layer_index[i][j−1]−1)).

In such a case, the syntax elements for signaling the number syntaxelement and the reference syntax elements could be as defined below.

for( i = 1; i ≦ vps_max_layers_minus1; i++ ) { Descriptornum_direct_ref_layers[ i ] u(v) for( j = 0; j < num_direct_ref_layers[ i]; j++) delta_ref_layer_index[ i ][ j ] u(v) }

An aspect of the embodiments relates to a method of determining codingrelationship for a multi-layer video stream 1 defining multiple layers10, 12, 14 of pictures 20, 22, 24. The method comprises determining anycoding relationship between the multiple layers 10, 12, 14. The methodalso comprises determining, for a layer 12 with layer index i of themultiple layers 10, 12, 14 and based on the coding relationship, atleast one direct decoding flag indicating a direct coding relationshipbetween the layer 12 with layer index i and a layer 10 with layer indexj of the multiple layers 10, 12, 14, i≠j. The method further comprisesassociating the at least one direct decoding flag with a codedrepresentation 2 of the multi-layer video stream 1.

FIG. 10 is a flow diagram illustrating an embodiment of this method ofdetermining coding relationship for a multi-layer video stream definingmultiple layers of pictures. The method generally starts in step S50,which comprises determining any coding relationship between the multiplelayers in the multi-layer video stream. The coding relationship ispreferably determined based on information of which layers referencepictures used for coding the pictures in a current layer belong to.Thus, by determining in which layer or layers the reference picturesused as coding basis for pictures in the current layer any codingrelationship between the various layers in the multi-layer video streamcan be determined in step S50.

The coding relationship determined for the multi-layer video stream instep S50 is then used in step S51 to determine, for a layer with layerindex i, at least one direct decoding flag indicating a direct codingrelationship between the layer with layer index i and a layer with layerindex j, where i is different from j. This means that the codingrelationship is employed to identify any direct coding relationship,i.e. direct dependency, between the current layer with layer index i andany other layers in the multi-layer video stream. Such direct dependencybetween layers is then indicated by the direct decoding flag(s)determined in step S51.

The method also comprises associating, in step S52, the at least onedirect decoding flag with a coded representation of the multi-layervideo stream. This associating in step S52 preferably, as has previouslybeen disclosed herein, involves including the direct decoding flag(s) ina parameter set, parameter set extension, other data structure of orassociated with the coded multi-layer video stream or in at least oneencoded representation of a slice in the coded multi-layer video stream.

The method then ends.

The method as disclosed in FIG. 10 could be implemented as a stand alonemethod of determining coding relationship for a multi-layer videostream. In an alternative approach, the method of FIG. 10 is usedtogether with the method of encoding a multi-layer video stream as shownin FIG. 9. In such a case, the steps S50 and S51 of FIG. 10 areperformed as a particular embodiment of determining the codingrelationship information in FIG. 9. In such a case, the methodpreferably starts in step S40 of FIG. 9 and continues to steps S50 andS51 of FIG. 10 and then returns to step S42 of FIG. 9. Step S43 of FIG.9 is then preferably performed as shown in step S52 of FIG. 10.

An aspect of the embodiments relates to a method of determining codingrelationship for a multi-layer video stream 1 defining multiple layers10, 12, 14 of pictures 20, 22, 24. The method comprises determining anumber syntax element defining a number of reference layers of themultiple layers 10, 12, 14 for a layer 12 with layer index i of themultiple layers 10, 12, 14. The method also comprises determining, foreach of the number of reference layers, a respective reference syntaxelement defining a layer index j of a layer 10 of the multiple layers10, 12, 14 that the layer 12 with layer index i directly depends on,j<i. The method further comprises associating the number syntax elementand the respective reference syntax elements with a coded representation2 of the multi-layer video stream 1.

FIG. 11 is a flow diagram illustrating an embodiment of this method ofdetermining coding relationship for a multi-layer video stream. Themethod starts in step S60 where a number syntax element is determinedfor a layer with layer index i in the multi-view video stream. Thisnumber syntax element represents the number of reference layers for thelayer with layer index i.

The number syntax element is preferably determined based on informationof which layers reference pictures used for coding the pictures in acurrent layer with layer index i belong to. Thus, by determining inwhich layer or layers the reference pictures used as coding basis forpictures in the current layer the number of such reference layers can bedefined and used for determining the number syntax element in step S60.

The method of FIG. 11 also comprises determining, in step S61, arespective reference syntax element for each of the number of referencelayers. Thus, if the number syntax element defines X reference layersthen X respective reference syntax elements are preferably determined instep S61. A reference syntax element as determined in step S61 defines alayer index j of a layer in the multi-layer video stream that the layerwith layer index i directly depends on. In a particular embodiment, ahierarchical layer dependency is used so that the j<i.

The number syntax element determined in step S60 and the referencesyntax elements determined in step S61 are then associated with a codedrepresentation of the multi-layer video stream in step S62. Thisassociating in step S62 preferably, as has previously been disclosedherein, involves including the number and reference syntax elements in aparameter set, parameter set extension, other data structure of orassociated with the coded multi-layer video stream or in at least oneencoded representation of a slice in the coded multi-layer video stream.

The method then ends.

The method as disclosed in FIG. 11 could be implemented as a stand alonemethod of determining coding relationship for a multi-layer videostream. In an alternative approach, the method of FIG. 11 is usedtogether with the method of encoding a multi-layer video stream as shownin FIG. 9. In such a case, the steps S60 and S61 of FIG. 11 areperformed as a particular embodiment of determining the codingrelationship information in FIG. 9. In such a case, the methodpreferably starts in step S40 of FIG. 9 and continues to steps S60 and651 of FIG. 11 and then returns to step S42 of FIG. 9. Step S43 of FIG.9 is then preferably performed as shown in step S62 of FIG. 11.

Simulation experiments have been conducted in order to determine thenumber of bits required for signaling layer dependencies. Threedifferent methods of signaling such layer dependencies have beencompared: the prior art layer dependency signaling disclosed in thepreviously mentioned document JCTVC-K1007 (referred to as K1007 in FIGS.20-22), layer dependency signaling as disclosed herein in whichnum_direct_ref_layers[i] and ref_layer_id[i][j] are in u(v) code insteadof u(6) code (referred to as u(v) in FIGS. 20-22) and layer dependencysignaling using direct_reference_flag[i][j] as disclosed herein(referred to as dependency_flag in FIGS. 20-22).

FIG. 20 illustrates the simulation results when each layer with layerindex i≧1 has one reference layer. FIG. 21 illustrates the simulationresults when each layer with layer index i≧2 has two reference layersand the layer with layer index i=1 has one reference layer, i.e. themaximum number of reference layers that are possible for this layer withlayer index i=1. FIG. 22 illustrates the simulation results when eachlayer with layer index i≧1 has the maximum number of reference layersthat are possible, i.e. a layer with layer index i=N has N referencelayers. The results in FIGS. 20-22 are depicted for 1 . . . 64 layers,with 64 corresponding to the maximum allowed number of layers.

The results in FIGS. 20-22 show that the u(v) method is always lowerthan for the K1007 method. Up to a certain threshold for the number oflayers (8 in FIG. 20, 21 in FIG. 21 and 64 in FIG. 22) thedependency_flag method has the lowest number of bits. The number of bitsfor the dependency_flag method is independent of the layer dependencyconfiguration.

An aspect of the embodiments defines a decoder configured to decode acoded multi-layer video stream 2 defining multiple layers 10, 12, 14 ofpictures 20, 22, 24, each layer 10, 12, 14 of the multiple layers 10,12, 14 having a respective layer identifier. The decoder comprises adecoding relationship information retriever configured to retrieve, fora layer 12 with a layer index of the multiple layers 10, 12, 14,decoding relationship information based on the coded multi-layer videostream 2. The decoding relationship information defining a respectivelayer index of any reference layer 10 of the multiple layers 10, 12, 14,on which the layer 12 directly depends. The decoder also comprises anindex-to-identifier mapping unit configured to map, for each referencelayer 10 and for the layer 12, its layer index to a layer identifierbased on mapping information of a hierarchical mapping relationshipbetween layer identifiers and layer indices. The mapping information isretrieved based on the coded multi-layer video stream 2. The decoderfurther comprises a decoding unit configured to decode a picture 22 ofthe layer 12 based on at least one previously decoded picture 20 in alayer 10 of the multiple layers 10, 12, 14 identified based on the layeridentifiers mapped from layer indices.

FIG. 12 is a schematic block diagram illustrating an implementationembodiment of a decoder 100 configured to decode a multi-layer videostream in which each layer has a respective layer identifier. Thedecoder 100 comprises a decoding relationship information retriever 110,also referred to as decoding relationship information retrieving unit ormeans. The decoding relationship information retriever 110 is configuredto retrieve decoding relationship information based on the codedmulti-layer video stream for a current layer having a layer index. Thedecoding relationship information retriever 110 is preferably configuredto retrieve the information as previously disclosed herein from a datastructure of or associated with the coded multi-layer video stream, suchas a parameter set, a parameter set extension, other data structure oran encoded representation of a slice.

The decoding relationship information as retrieved by the decodingrelationship information retriever 110 defines a respective layer indexof any reference layer in the multi-layer video stream, on which thecurrent layer directly depends.

An index-to-identifier mapping unit 120, also denotedindex-to-identifier mapper or mapping means, is configured to map alayer index to a layer identifier for each reference layer and for thecurrent layer based on mapping information of a hierarchical mappingrelationship between layer identifiers and indices. Theindex-to-identifier mapping unit 120 is configured to retrieve themapping information based on the coded multi-layer video stream,preferably from a same data structure from which the decodingrelationship information retriever 110 retrieves the decodingrelationship information.

The layer identifiers as obtained from the index-to-identifier mappingunit 120 are used by a decoding unit 130, also referred to as picturedecoder or decoding means, when decoding a picture of the current layerbased on at least one previously decoded picture in a layer identifiedbased on the layer identifiers.

In an embodiment, the decoding relationship information is input to theindex-to-identifier mapping unit 120, which then maps the layerindex/indices of the layer/layers the current layer directly depends on.

Thus, in this embodiment only such layer index/indices are mappedtogether with the layer index of the current layer into respective layeridentifiers.

In an alternative embodiment, the index-to-identifier mapping unit 120is configured to map all layer indices that are signaled in orassociated with the coded multi-layer video stream into respective layeridentifiers. In such a case, the decoding unit 130 preferably identifiesthose layer identifiers among the mapped layer identifiers that are ofrelevance for decoding the pictures in the current layer based on thedecoding relationship information from the decoding relationshipinformation retriever 110.

Thus, the decoding relationship information and the mapping informationthereby together enable identification and determination of the layeridentifiers of any reference layers comprising pictures based on whichpictures in the current layer should be decoded.

In an embodiment the index-to-identifier mapping unit 120 is configuredto retrieve a flag vps_nuh_layer_id_present_flag based on the codedmulti-layer video stream. The index-to-identifier mapping unit 120 isalso configured to set, for each reference layer and for the layer andif vps_nuh_layer_id_present_flag=0, its layer identifier equal to itslayer index. The index-to-identifier mapping unit 120 is furtherconfigured to retrieve, for each reference layer and for the layer andif vps_nuh_layer_id_present_flag=1, its layer identifier from a vectorlayer_id_in_nuh[i], iε[1, vps_max_layers_minus1], whereinvps_max_layers_minus1+1 indicates a maximum number of layers andlayer_id_in_nuh[i] indicates a layer identifier for a layer with layerindex i.

An aspect of the embodiments defines an encoder configured to encode amulti-layer video stream 1 defining multiple layers 10, 12, 14 ofpictures 20, 22, 24, each layer 10, 12, 14 of the multiple layers 10,12, 14 having a respective layer identifier. The encoder comprises amapping unit configured to hierarchically map, for each layer 10, 12, 14of the multiple layers 10, 12, 14, a layer identifier of the layer 10,12, 14 to a layer index based on coding dependencies between themultiple layers 10, 12, 14. The encoder also comprises an informationdeterminer configured to determine coding relationship informationdefining a respective layer index of any reference layer 10 of themultiple layers 10, 12, 14, on which a layer 12 of the multiple layers10, 12, 14 directly depends. The encoder further comprises an encodingunit configured to generate a coded multi-layer video stream 2 byencoding the pictures 20, 22, 24 of the multiple layers 10, 12, 14 basedon the coding dependencies and an associating unit configured toassociate the coding relationship information with the coded multi-layervideo stream 2.

FIG. 15 is a schematic block diagram of an embodiment of an encoder 400configured to encode a multi-layer video stream in which each layer hasa respective layer identifier. The encoder 400 comprises a mapping unit410, also referred to as mapper or mapping means. The mapping unit 410is configured to hierarchically map a layer identifier for each layer ofthe multi-layer video stream to a layer index based on codingdependencies between the multiple layers in the multi-layer videostream. This means that information about which layer(s) that is (are)reference layer(s) for (an) other layer(s) is used by the mapping unit410 to assign layer indices to the layers.

The encoder 400 also comprises an information determiner 420, alsodenoted information determining unit or means. The informationdeterminer 420 is configured to determine coding relationshipinformation defining a respective layer index of any reference layerbased on which a current layer directly depends on.

An encoding unit 430, also referred to as picture encoder or encodingmeans, is configured to generate a coded multi-layer video stream byencoding the pictures in the multiple layers based on the codingdependencies. An associating unit 440, also referred to as associator orassociating means, is configured to associate the coding relationshipinformation with the coded multi-layer video stream, as previouslydisclosed herein, such as in a parameter set, parameter set extension,other data structure or encoded representation of a slice.

The associating unit 440 is preferably also configured to associatemapping information defining a mapping information defining ahierarchical mapping relationship between the layer identifiers and thelayer indices with the coded multi-layer video stream.

Other aspects of the embodiments relate to devices for determiningdecoding or encoding relationships for a (coded) multi-layer videostream. Such devices could be provided as separate entities or could beimplemented as part of the decoder 100 of FIG. 12 or the encoder 400 ofFIG. 15.

An aspect of the embodiments defines to a device for determiningdecoding relationship for a coded multi-layer video stream 2 definingmultiple layers 10, 12, 14 of pictures 20, 22, 24. The device comprisesa flag retriever configured to retrieve, based on the coded multi-layervideo stream 2, at least one direct decoding flag indicating a directcoding relationship between a layer 12 with layer index i of themultiple layers 10, 12, 14 and a layer 10 with layer index j of themultiple layers 10, 12, 14, i≠j. The device also comprises aninformation determiner configured to determine information defining anydecoding relationship between the multiple layers 10, 12, 14 based onthe at least one direct decoding flag.

FIG. 13 is an illustration of an embodiment of such a device 200 fordetermining decoding relationship. The device 200 comprises a flagretriever 210, also referred to as flag retrieving unit or means. Theflag retriever 210 is configured to retrieve at least one directdecoding flag based on the coded multi-layer video stream, such as froma parameter set, parameter set extension, other data structure of anencoded representation of a slice in coded multi-layer video stream. Theretrieved at least one direct decoding flag indicates a direct codingrelationship between a current layer with layer index i and a layer withlayer index j of the multi-layer video stream, where the index i isdifferent from the index j.

The device 200 also comprises an information determiner 220, alsoreferred to as information determining unit or means. The informationdeterminer 220 is configured to determine information defining anydecoding relationship between the multiple layers in the multi-layervideo stream based on the at least one direct decoding flag retrieved bythe flag retriever 210.

In an embodiment the flag retriever 210 is configured to retrieve, forthe layer with layer index i, a respective direct dependency flag foreach layer index j based on the coded multi-layer video stream. In thisembodiment, j<i and the direct dependency flag indicates whether thelayer with layer index j is a direct reference layer for the layer withlayer index i. The information determiner 220 is preferably configuredto determine information defining any layer with layer index j<i, onwhich the layer with layer index i depends, based on the directdependency flags.

In an implementation example the flag retriever 210 is configured toretrieve, for the layer with layer index i, a respective directdependency flag direct_dependency_flag[i][j] for each layer index jbased on the coded multi-layer video stream. The information determiner220 is preferably configured to determine a dependency flag setdependency_flag[i] asdependency_flag[i]=direct_dependency_flag[i]+sum_(j=0 . . . i−1)(direct_dependency_flag[i][j]×dependency_flag[j]).The dependency flag set or the dependency flags dependency_flag[i][j]then constitute(s) in this implementation example the informationdefining any decoding relationship between the multiple layers.

In another embodiment the flag retriever 210 is configured to retrieve,for the layer with layer index i, a respective direct reference flag foreach layer index j based on the coded multi-layer video stream. In thisembodiment, j>i and the direct reference flag indicates whether thelayer with layer index i is a direct reference layer for the layer withlayer index j. The information determiner 220 is, In this embodiment,preferably configured to determine information defining any layer withlayer index j>i for which the layer with layer index i is a referencelayer based on the direct reference flags.

In another implementation example the flag retriever 210 is configuredto retrieve, for the layer with layer index i, a respective directreference flag direct_reference_flag[i][j] for each layer index j basedon the coded multi-layer video stream. The information determiner 220is, in this example, preferably configured to determine a reference flagset reference_flag[i] asreference_flag[i]=direct_reference_flag[i]+sum_(j=i+1 . . . imax)(direct_reference_flag[i][j]×reference_flag[j]).The reference flag set or the reference flags reference_flag[i][j] thenconstitute(s) in this implementation example the information definingany decoding relationship between the multiple layers.

The device for determining decoding relationship 200 in FIG. 13 is in anembodiment a particular implementation example of the decodingrelationship information retriever 110 of FIG. 12.

Another aspect of the embodiments defines a device for determiningdecoding relationship for a coded multi-layer video stream 2 definingmultiple layers 10, 12, 14 of pictures 20, 22, 24. The device optionallycomprises a length calculator configured to calculate a length of anumber syntax element. The device comprises an element retrieverconfigured to retrieve, based on the coded multi-layer video stream 2,the number syntax element defining a number of reference layers 10 ofthe multiple layers 10, 12, 14 for a layer 12 with layer index i of themultiple layers 10, 12, 14, optionally based on the length of the numbersyntax element. The element retriever is further configured to retrieve,based on the coded multi-layer video stream 2 and for each of the numberof reference layers, a respective reference syntax element defining alayer index j of a layer 10 of the multiple layers 10, 12, 14 that thelayer 12 with layer index i directly depends on, j<i.

FIG. 14 is a schematic block diagram of such a device 300 fordetermining decoding relationship. The device 300 optionally comprises alength calculator 310, also denoted length calculating unit or means.The length calculator 310 is preferably configured to calculate a lengthof a number syntax element defining a number of reference layer in themulti-layer video steam for a current layer with layer index i. Anelement retriever 320, also denoted element retrieving unit or means, ofthe device 300 is configured to retrieve the number syntax element basedon the coded multi-layer video stream, such as from a parameter set,parameter set extension, other data structure or encoded representationof a slice. In a particular embodiment, the element retriever 320 usesinformation of the length of the number syntax element, preferably asdetermined by the length calculator 310, in order to identify theportion of the relevant number-syntax-element-carrying data structurethat corresponds to the number syntax element.

The value of the retrieved number syntax element is also used by theelement retriever 320 to determine how many reference syntax elements toretrieve for the current layer with layer index i. Thus, the elementretriever 320 retrieves a respective reference syntax element for eachof the number of reference layers as defined by the number syntaxelement. The element retriever 320 preferably retrieves the referencesyntax element from parameter set, parameter set extension, other datastructure or encoded representation of a slice. In a particularembodiment, the element retriever 320 is configured to retrieve thereference syntax element from the same data structure that alsocomprises the number syntax element.

Each reference syntax element retrieved by the element retriever 320defines a layer index j of a reference layer that the current layer withlayer index i directly depends on. In a particular embodiment, ahierarchical layer dependency is used so that j<i.

In an embodiment the length calculator 310 is configured to calculatethe length of the number syntax element based on the layer index i, i.e.based on the value of the layer index i.

In a particular embodiment the length calculator 310 is configured tocalculate the length of the number syntax element as ceil(log 2(i+1)).

In another embodiment the length calculator 310 is configured tocalculate the length of the number syntax element based on informationof a maximum possible number of layers in the coded multi-layer videostream.

In another particular embodiment the length calculator 310 is configuredto calculate the length of the number syntax element as ceil(log2(vps_max_layers_minus1+1)).

In a further embodiment the length calculator 310 is configured tocalculate the length of the number syntax element based on informationof a maximum value for a layer identifier in the coded multi-layer videostream.

In a further particular embodiment the length calculator 310 isconfigured to calculate the length of the number syntax element asceil(log 2(vps_max_layer_id−1)).

In an embodiment the optional length calculator 310 is configured tocalculate a respective length of the respective reference syntaxelement.

In particular embodiment the length calculator 310 is configured tocalculate the respective length of the respective reference syntaxelement based on the layer index i.

In another particular embodiment the length calculator 310 is configuredto calculate the respective length of the reference syntax element asceil(log 2(i−ref layer index[i][j−1]−1)). In such a case, the elementretriever 320 is preferably configured to retrieve, based on the codedmulti-layer video stream and for each layer j of the number of referencelayers, a respective delta reference syntax elementdelta_ref_layer_index[i][j], j<i. The device 300 preferably comprises anoptional index determiner 330 configured to determine, for each layer jof the number of reference layers, the layer index of the j-th layer thelayer with layer index i directly depends on as ref layerindex[i][j]=[i][j−1]+1+delta_ref_layer_index[i][j] with ref layerindex[i][−1]=−1, 0<j<i.

The device for determining decoding relationship 300 in FIG. 14 is in anembodiment a particular implementation example of the decodingrelationship information retriever 110 of FIG. 12.

An aspect of the embodiments defines a device for determining codingrelationship for a multi-layer video stream 1 defining multiple layers10, 12, 14 of pictures 20, 22, 24. The device comprises a relationshipdeterminer configured to determine any coding relationship between themultiple layers 10, 12, 14. The device also comprises a flag determinerconfigured to determine, for a layer 12 with layer index i of themultiple layers 10, 12, 14 and based on the coding relationship, atleast one direct decoding flag indicating a direct coding relationshipbetween the layer 12 with layer index i and a layer 10 with layer indexj of the multiple layers 10, 12, 14, i≠j. An associating unit isconfigured to associate the at least one direct decoding flag with acoded representation 2 of the multi-layer video stream 1.

FIG. 16 shows a block diagram of an embodiment of this device 500 fordetermining coding relationship for a multi-layer video stream. Thedevice 500 comprises a relationship determiner 510, also referred to asrelationship determining unit or means. The relationship determiner 510is configured to determine any coding relationship between the multiplelayers in the multi-layer video stream. The relationship determiner 510preferably performs this determination as previously disclosed hereinbased on information of which layers that are used as reference layersfor other layers in the multi-layer video stream. The device 500 alsocomprises a flag determiner 520, also referred to as flag determiningunit or means. The flag determiner 520 is configured to determine atleast one direct decoding flag for a current layer with layer index iand based on the coding relationship as determined by the relationshipdeterminer 510. A direct decoding flag indicates a direct codingrelationship between the current layer with layer index i and a layerwith layer index j≠i.

The device 500 further comprises an associating unit 530, also referredto as associator or associating means, configured to associate the atleast one decoding flag with a coded representation of the multi-layervideo stream, such as inserting the at least one decoding flag in aparameter set, parameter set extension, other data structure or encodedrepresentation of a slice.

The device for determining coding relationship 500 in FIG. 16 is in anembodiment a particular implementation example of the informationdeterminer 420 of FIG. 15.

An aspect of the embodiments defines a device for determining codingrelationship for a multi-layer video stream 1 defining multiple layers10, 12, 14 of pictures 20, 22, 24. The device comprises a number syntaxelement determiner configured to determine a number syntax elementdefining a number of reference layers 10 of the multiple layers 10, 12,14 for a layer 12 with layer index i of the multiple layers 10, 12, 14.The device also comprises a reference syntax element determinerconfigured to determine, for each of the number of reference layers 10,a respective reference syntax element defining a layer index j of alayer 10 of the multiple layers 10, 12, 14 that the layer 12 with layerindex i directly depends on, j<i. The device further comprises anassociating unit configured to associate the number syntax element andthe respective reference syntax elements with a coded representation 2of the multi-layer video stream 1.

FIG. 17 shows a block diagram of an embodiment of this device 600 fordetermining coding relationship for a multi-layer video stream. Thedevice 600 comprises number syntax element determiner 610, also denotednumber syntax element determining unit or means. The number syntaxelement determiner 610 is configured to determine a number syntaxelement defining a number of reference layer for layer with layer indexi. The number syntax element determiner 610 preferably determines thenumber syntax elements based on information of which reference layers,if any, reference pictures, based on which pictures in the current layerwith layer index i, belong.

The device 600 also comprises a reference syntax element determiner 620,sometimes referred to as reference syntax element determining unit ormeans. The reference syntax element determiner 620 is configured todetermine a respective reference syntax element for each of the numberof reference layers as defined by the number syntax element. A referencesyntax element as determined by the reference syntax element determiner620 defines a layer index j of a reference layer of the multi-layervideo stream that the current layer with layer index i directly dependson. In a particular embodiment, a hierarchical layer dependency is usedso that j<i.

An associating unit 630, also denoted associator or associating means,of the device 600 is configured to associate the number syntax elementand the respective reference syntax elements with a coded representationof the multi-layer video stream. The number and reference syntaxelements could be inserted in a same or different parameter sets,parameter set extensions, other data structures or encodedrepresentations of slices.

The device for determining coding relationship 600 in FIG. 17 is in anembodiment a particular implementation example of the informationdeterminer 420 of FIG. 15.

The decoder 100 in FIG. 12, the devices 200, 300 for determiningdecoding relationship in FIGS. 13 and 14, the encoder 400 of FIG. 15 andthe devices 500, 600 for determining coding relationship in FIGS. 16 and17 may be implemented in hardware. There are numerous variants ofcircuitry elements that can be used and combined to achieve thefunctions of the units of the decoder, encoder or devices in FIGS.12-17. Such variants are encompassed by the embodiments. Particularexamples of hardware implementation include implementation in digitalsignal processor (DSP) hardware and integrated circuit technology,including both general-purpose electronic circuitry andapplication-specific circuitry.

Alternatively, the decoder 100 in FIG. 12, the devices 200, 300 fordetermining decoding relationship in FIGS. 13 and 14, the encoder 400 ofFIG. 15 and the devices 500, 600 for determining coding relationship inFIGS. 16 and 17 may be implemented at least partly in software. Such animplementation is shown in FIG. 18. The decoder, encoder or device 700of FIGS. 12-17 then comprises a processor 710 configured to process codemeans 740 of a computer program 730 stored in a computer readablemedium, represented by a memory 720 in FIG. 18. The code means 740causes, when run on the processor 710, the processor 710 to perform thefunctions of the units of the decoder, encoder or devices of FIGS.12-17.

The processor 710 could be a general purpose or specially adaptedcomputer, processor or microprocessor, such as a central processing unit(CPU). The software includes computer program code elements or softwarecode portions effectuating the operation of at least the decodingrelationship information retriever 110, the index-to-identifier mappingunit 120 and the decoding unit 130 of FIG. 12, and/or the flag retriever210 and the information determiner 220 of FIG. 13, and/or the optionallength calculator 310, the element retriever 320 and the optional indexdeterminer 330 of FIG. 14, and/or the mapping unit 410, the informationdeterminer 420, the encoding unit 430 and the associating unit 440 ofFIG. 15, and/or the relationship determiner 510, the flag determiner 520and the associating unit 530 of FIG. 16, and/or the number syntaxelement determiner 610, the reference syntax element determiner 620 andthe associating unit 630 of FIG. 17.

The computer program 730 may be stored in whole or part, on or in one ormore suitable volatile computer readable media or data storage means720, such as RAM, or one or more non-volatile computer readable media ordata storage means, such as magnetic disks, CD-ROMs, DVD disks, harddiscs, in ROM or flash memory. The data storage means 720 can be a localdata storage means or is remotely provided, such as in a data server.The software may thus be loaded into the operating memory of a computeror equivalent processing system for execution by a processor. Thecomputer/processor does not have to be dedicated to only execute theabove-described functions but may also execute other software tasks. Anon-limiting example of program code used to define the network nodeinclude single instruction multiple data (SIMD) code.

The reference number 700 in FIG. 18, thus, indicates the decoder,encoder or device of FIGS. 12-17 as implemented at least partly insoftware.

The decoder 100 of FIG. 12, the device 200 for determining decodingrelationship in FIG. 13 and/or the device 300 for determining decodingrelationship in FIG. 14 can advantageously be arranged in a user deviceor terminal 53 as shown in FIG. 19. This user device 53 could thenrepresent a set-top-box, a computer, a mobile device, such as mobiletelephone, laptop or tablet computer, etc. The device 200 of FIG. 13 orthe device of 300 of FIG. 14 is advantageously implemented in thedecoder 100 of FIG. 12.

The user device 53 may, in addition to a decoder 54, such as a decoder100 shown in FIG. 12 or another decoder comprising a device 200 of FIG.13 or a device 300 of FIG. 14, comprise a decoded picture buffer (DPB)55, in which the user device 53 stores decoded pictures as generated bythe decoder 54. These decoded pictures are temporarily stored in the DPB55 i) to be used as reference pictures when decoding subsequent,according to the decoding order, pictures of the multi-layer videostream and/or ii) until they are output, such as output for display on adisplay 56 or screen of or connected to the user device 53.

The decoder 100 of FIG. 12, the device 200 for determining decodingrelationship in FIG. 13 and/or the device 300 for determining decodingrelationship in FIG. 14 may, also or alternatively, be implemented in anetwork node 52 or network element. In such a case, the network node 52or network element could perform, e.g. selective forwarding of layers ofthe coded multi-layer video stream, based on and thereby considering thelayer dependencies.

The encoder 400 of FIG. 15, the device 500 for determining codingrelationship in FIG. 16 and/or the device 600 for determining codingrelationship in FIG. 17 can advantageously be arranged in a user deviceor terminal 50 as shown in FIG. 19. This user device 50 could thenrepresent a video camera, a computer, a mobile device, such as mobiletelephone, laptop or tablet computer, etc. The device 500 of FIG. 16 orthe device of 600 of FIG. 17 is advantageously implemented in theencoder 400 of FIG. 15.

The encoder 51 implemented in the user device 50 could be the encoder400 shown in FIG. 15 or another encoder comprising a device 500 of FIG.16 or a device 600 of FIG. 17.

Pictures 20, 22, 24 of multiple layers 10, 12, 14 in a multi-layer videostream 1 are input to the encoder 51 of the user device 50 to generateand output a coded multi-layer video stream 2, typically comprising NALunits containing respective encoded representations of the slices in thepictures or parameter sets generated for the multi-layer video stream 1.

The coded multi-layer video stream 2 is preferably transmitted,wirelessly or using a wired connection, from the user device 50 towardsthe user device 53, where the decoder 54 decodes the coded multi-layervideo stream 2 to generate decoded pictures that can be shown on thedisplay 56. Non-limiting examples of transport protocols that could beused in a network in order to transmit a coded multi-layer video stream2 and/or data packets carrying NAL units that are sent out-of-bandinclude RTP, Dynamic Adaptive Streaming over the Hypertext TransferProtocol (DASH), MPEG2 Transport Stream (TS). The network is preferablya wireless network, such as a mobile (broadband) network.

The coded multi-layer video stream 2 may be transmitted from theencoding user device 50 to the decoding user device 53 through one ormore network nodes 52. Such a network node 52 could optionally processthe coded multi-layer video stream 2, such as transcode the multi-layervideo stream, or remove one or more layers of the coded multi-layervideo stream 2 to form a coded multi-layer video sub-stream 3. Such alayer discarding could be due to bit rate adaptation, e.g. in the caseof network congestion; format adaptation, e.g. in the case the decodinguser device 53 can only decode or display a certain maximum resolution;or 2D/3D adaptation, e.g. in the case the decoding user device 53 canonly decode or display a certain number of views.

The present embodiments are in particular applicable to HEVC and othervideo coding standards that are capable of coding and decodingmulti-layer video, such as multi-view video, 3D video or scalable video.

Modifications and other variants of the described embodiments will cometo mind to one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the embodiments are not to belimited to the specific examples disclosed and that modifications andother variants are intended to be included within the scope of thisdisclosure. In particular, embodiments as disclosed herein may becombined where technically feasible.

1-25. (canceled)
 26. A method, in a processing circuit, of determining adecoding relationship for a digitally coded multi-layer video streamdefining multiple layers of pictures, said method comprising:retrieving, based on said digitally coded multi-layer video stream, atleast one direct decoding flag indicating a direct coding relationshipbetween a layer with layer index i of said multiple layers and a layerwith layer index j of said multiple layers, where i is not equal to j;and determining information defining any decoding relationship betweensaid multiple layers based on said at least one direct decoding flag.27. The method of claim 26, wherein retrieving said at least one directdecoding flag comprises retrieving, for said layer with layer index i, arespective direct dependency flag for each layer index j based on saidcoded multi-layer video stream, wherein j<i and said direct dependencyflag indicates whether said layer with layer index j is a directreference layer for said layer with layer index i; and whereindetermining said information comprises determining information definingany layer with layer index j<i which said layer with layer index idepends on, based on said direct dependency flags.
 28. The method ofclaim 26, wherein retrieving said at least one direct decoding flagcomprises retrieving, from a video parameter set or video parameter setextension associated with said coded multi-layer video stream, said atleast one direct decoding flag indicating said direct codingrelationship between said layer with layer index i and said layer withlayer index j.
 29. A method, in a processing circuit, of decoding adigitally coded multi-layer video stream defining multiple layers ofpictures, each layer of said multiple layers having a respective layeridentifier, said method comprising: retrieving, for a layer with a layerindex of said multiple layers, decoding relationship information basedon said digitally coded multi-layer video stream, said decodingrelationship information defining a respective layer index of anyreference layer of said multiple layers on which said layer directlydepends; mapping, for each reference layer and for said layer, its layerindex to a layer identifier based on mapping information of ahierarchical mapping relationship between layer identifiers and layerindices, wherein said mapping information is retrieved based on saiddigitally coded multi-layer video stream; and decoding a picture of saidlayer based on at least one previously decoded picture in a layer ofsaid multiple layers identified based on said layer identifiers mappedfrom layer indices.
 30. The method of claim 29, wherein retrievingdecoding relationship information comprises: retrieving, based on saidcoded multi-layer video stream, at least one direct decoding flagindicating a direct coding relationship between a layer with layer indexi of said multiple layers and a layer with layer index j of saidmultiple layers, where i is not equal to j; and determining saiddecoding relationship information based on said at least one directdecoding flag.
 31. The method of claim 29, wherein mapping its layerindex to a layer identifier comprises: retrieving a flagvps_nuh_layer_id_present_flag based on said coded multi-layer videostream; setting, for each reference layer and for said layer and ifvps_nuh_layer_id_present_flag=0, its layer identifier equal to its layerindex; and retrieving, for each reference layer and for said layer andif vps_nuh_layer_id_present_flag=1, its layer identifier from a vectorlayer_id_in_nuh[i], iε[1, vps_max_layers_minus1], whereinvps_max_layers_minus1+1 indicates a maximum number of layers andlayer_id_in_nuh[i] indicates a layer identifier for a layer with layerindex i.
 32. A device for determining decoding relationship for adigitally coded multi-layer video stream defining multiple layers ofpictures, said device comprising: a flag retriever configured toretrieve, based on said coded multi-layer video stream, at least onedirect decoding flag indicating a direct coding relationship between alayer with layer index i of said multiple layers and a layer with layerindex j of said multiple layers, where i is not equal to j; and aninformation determiner configured to determine information defining anydecoding relationship between said multiple layers based on said atleast one direct decoding flag.
 33. The device of claim 32, wherein saidflag retriever is configured to retrieve, for said layer with layerindex i, a respective direct dependency flag for each layer index jbased on said coded multi-layer video stream, wherein j is less than iand wherein said direct dependency flag indicates whether said layerwith layer index j is a direct reference layer for said layer with layerindex i; and wherein said information determiner is configured todetermine information defining any layer with layer index j less than iwhich said layer with layer index i depends on, based on said directdependency flags.
 34. The device of claim 32, wherein said flagretriever is configured to retrieve, from a video parameter set or videoparameter set extension associated with said coded multi-layer videostream, said at least one direct decoding flag indicating said directcoding relationship between said layer with layer index i and said layerwith layer index j.
 35. A device for determining a decoding relationshipfor a digitally coded multi-layer video stream defining multiple layersof pictures, said device comprising a non-transitory computer readablemedium and an associated processor configured to process computerprogram instructions stored in the computer readable medium, wherein thestored computer program instructions, when run on said processor, areconfigured to cause the processor to: retrieve, based on said codedmulti-layer video stream, at least one direct decoding flag indicating adirect coding relationship between a layer with layer index i of saidmultiple layers and a layer with layer index j of said multiple layers,where i is not equal to j; and determine information defining anydecoding relationship between said multiple layers based on said atleast one direct decoding flag.
 36. A decoder device configured todecode a coded multi-layer video stream defining multiple layers ofpictures, each layer of said multiple layers having a respective layeridentifier, said decoder comprising: a decoding relationship informationretriever configured to retrieve, for a layer with a layer index of saidmultiple layers, decoding relationship information based on said codedmulti-layer video stream, said decoding relationship informationdefining a respective layer index of any reference layer of saidmultiple layers, on which said layer directly depends; anindex-to-identifier mapping unit configured to map, for each referencelayer and for said layer, its layer index to a layer identifier based onmapping information of a hierarchical mapping relationship between layeridentifiers and layer indices, said mapping information is retrievedbased on said coded multi-layer video stream; and a decoding unitconfigured to decode a picture of said layer based on at least onepreviously decoded picture in a layer of said multiple layers identifiedbased on said layer identifiers mapped from layer indices.
 37. Thedecoder device of claim 36, wherein said decoding relationshipinformation retriever comprises: a flag retriever configured toretrieve, based on said coded multi-layer video stream, at least onedirect decoding flag indicating a direct coding relationship between alayer with layer index i of said multiple layers and a layer with layerindex j of said multiple layers, where i is not equal to j; and aninformation determiner configured to determine said decodingrelationship information based on said at least one direct decodingflag.
 38. The decoder device of claim 36, wherein saidindex-to-identifier mapping unit is configured to a) retrieve a flagvps_nuh_layer_id_present_flag based on said coded multi-layer videostream; b) set, for each reference layer and for said layer and ifvps_nuh_layer_id_present_flag=0, its layer identifier equal to its layerindex; and c) retrieve, for each reference layer and for said layer andif vps_nuh_layer_id_present_flag=1, its layer identifier from a vectorlayer_id_in_nuh[i], iε[1, vps_max_layers_minus1], whereinvps_max_layers_minus1+1 indicates a maximum number of layers andlayer_id_in_nuh[i] indicates a layer identifier for a layer with layerindex i.
 39. A decoder device configured to decode a coded multi-layervideo stream defining multiple layers of pictures, each layer of saidmultiple layers having a respective layer identifier, said decodercomprising a non-transitory computer readable medium and an associatedprocessor configured to process computer program instructions stored inthe computer readable medium, wherein the stored computer programinstructions, when run on said processor, are configured to cause theprocessor to: retrieve, for a layer with a layer index of said multiplelayers, decoding relationship information based on said codedmulti-layer video stream, said decoding relationship informationdefining a respective layer index of any reference layer of saidmultiple layers, on which said layer directly depends; map, for eachreference layer and for said layer, its layer index to a layeridentifier based on mapping information of a hierarchical mappingrelationship between layer identifiers and layer indices, said mappinginformation is retrieved based on said coded multi-layer video stream;and decode a picture of said layer based on at least one previouslydecoded picture in a layer of said multiple layers identified based onsaid layer identifiers mapped from layer indices.
 40. A method ofdetermining coding relationship for a multi-layer video stream definingmultiple layers of pictures, said method comprising: determining anycoding relationship between said multiple layers; determining, for alayer with layer index i of said multiple layers and based on saidcoding relationship, at least one direct decoding flag indicating adirect coding relationship between said layer with layer index i and alayer with layer index j of said multiple layers, where i is not equalto j; and associating said at least one direct decoding flag with acoded representation of said multi-layer video stream.
 41. A method ofencoding a multi-layer video stream defining multiple layers ofpictures, each layer of said multiple layers having a respective layeridentifier, said method comprises: hierarchically mapping, for eachlayer of said multiple layers a layer identifier of said layer to alayer index based on coding dependencies between said multiple layers;determining coding relationship information defining a respective layerindex of any reference layer of said multiple layers, on which a layerof said multiple layers directly depends; generating a coded multi-layervideo stream by encoding said pictures of said multiple layers based onsaid coding dependencies; and associating said coding relationshipinformation with said coded multi-layer video stream.
 42. The method ofclaim 41, wherein determining said coding relationship informationcomprises: determining any coding relationship between said multiplelayers and determining, for a layer with layer index i of said multiplelayers and based on said coding relationship, at least one directdecoding flag indicating a direct coding relationship between said layerwith layer index i and a layer with layer index j of said multiplelayers, where i is not equal to j; and associating said codingrelationship information comprises associating said at least one directdecoding flag with said coded multi-layer video stream.
 43. A device fordetermining a coding relationship for a multi-layer video streamdefining multiple layers of pictures, said device comprising: arelationship determiner configured to determine any coding relationshipbetween said multiple layers; a flag determiner configured to determine,for a layer with layer index i of said multiple layers and based on saidcoding relationship, at least one direct decoding flag indicating adirect coding relationship between said layer with layer index i and alayer with layer index j of said multiple layers, where i is not equalto j; and an associating unit configured to associate said at least onedirect decoding flag with a coded representation of said multi-layervideo stream.
 44. A device for determining a coding relationship for amulti-layer video stream defining multiple layers of pictures, saiddevice comprising a non-transitory computer readable medium and anassociated processor configured to process computer program instructionsstored in the computer readable medium, wherein the stored computerprogram instructions, when run on said processor, are configured tocause the processor to: determine any coding relationship between saidmultiple layers; determine, for a layer with layer index i of saidmultiple layers and based on said coding relationship, at least onedirect decoding flag indicating a direct coding relationship betweensaid layer with layer index i and a layer with layer index j of saidmultiple layers, where i is not equal to j; and associate said at leastone direct decoding flag with a coded representation of said multi-layervideo stream.
 45. An encoder device configured to digitally encode amulti-layer video stream defining multiple layers of pictures, eachlayer of said multiple layers having a respective layer identifier, saidencoder device comprising: a mapping unit configured to hierarchicallymap, for each layer of said multiple layers, a layer identifier of saidlayer to a layer index based on coding dependencies between saidmultiple layers; an information determiner configured to determinecoding relationship information defining a respective layer index of anyreference layer of said multiple layers, on which a layer of saidmultiple layers directly depends; an encoding unit configured togenerate a coded multi-layer video stream by encoding said pictures ofsaid multiple layers based on said coding dependencies; and anassociating unit configured to associate said coding relationshipinformation with said coded multi-layer video stream.
 46. The encoderdevice of claim 45, wherein said information determiner comprises: arelationship determiner configured to determine any coding relationshipbetween said multiple layers; and a flag determiner configured todetermine, for a layer with layer index i of said multiple layers andbased on said coding relationship, at least one direct decoding flagindicating a direct coding relationship between said layer with layerindex i and a layer with layer index j of said multiple layers, where iis not equal to j; and said associating unit is configured to associatesaid at least one direct decoding flag with said coded multi-layer videostream.
 47. An encoder device configured to encode a multi-layer videostream defining multiple layers of pictures, each layer of said multiplelayers having a respective layer identifier, said encoder comprising anon-transitory computer readable medium and an associated processorconfigured to process computer program instructions stored in thecomputer readable medium, wherein the stored computer programinstructions, when run on said processor, are configured to cause theprocessor to: hierarchically map, for each layer of said multiplelayers, a layer identifier of said layer to a layer index based oncoding dependencies between said multiple layers; determine codingrelationship information defining a respective layer index of anyreference layer of said multiple layers, on which a layer of saidmultiple layers directly depends; generate a coded multi-layer videostream by encoding said pictures of said multiple layers based on saidcoding dependencies; and associate said coding relationship informationwith said coded multi-layer video stream.
 48. A user device comprisingthe decoder device of claim
 36. 49. A user device comprising the deviceof claim
 43. 50. A network node comprising the decoder device of claim36.